[native] Replace std::mutex with pthread_mutex_t in the CoreCLR host

[simonrozsival]

Part of #12533 (drop libc++ from the CoreCLR host). Stacked on top of #12534.

std::mutex is a thin wrapper over pthread_mutex_t — it holds one as its only member and its lock/unlock forward straight to pthread_mutex_lock/pthread_mutex_unlock. Using it in the host buys us nothing over the primitive itself, but it drags <mutex> into six translation units.

This replaces it with pthread_mutex_t directly.

What

• std::mutex → pthread_mutex_t x = PTHREAD_MUTEX_INITIALIZER
• std::lock_guard → explicit pthread_mutex_lock (&x) / pthread_mutex_unlock (&x), with the unlock placed immediately before any early return
• StartupAwareLock now takes a pthread_mutex_t&
• #include <mutex> removed; explicit #include <pthread.h> added where the type is used

No locked region changed shape. Every hunk is a type change, a lock call, or an added unlock — there is no restructured logic to re-verify.

File	Lock
clr/host/assembly-store.cc + include/host/assembly-store.hh	state_lock
clr/host/fastdev-assemblies.cc + include/host/fastdev-assemblies.hh	override_dir_lock (DEBUG-only)
clr/include/runtime-base/monodroid-dl.hh	dso_handle_write_lock
clr/include/runtime-base/startup-aware-lock.hh	takes pthread_mutex_t&
common/include/runtime-base/timing.hh	sequence_lock

timing.hh is the only file outside clr/. It used std::mutex directly, so it cannot be skipped; MonoVM builds clean with the change and mono/ is otherwise untouched (MonoVM has its own xamarin::android::mutex in mono/shared/cppcompat.hh, which already wrapped pthreads and is left alone).

Effect

This removes the last #include <mutex> in the repository:

	files including <mutex>	std::mutex / std::lock_guard uses
before	6	13
after	0	0

It also drops eight undefined references, since lock (), unlock () and the destructor are out-of-line in libc++ rather than header-inlined:

symbol	before	after
std::__ndk1::mutex::lock()	3	0
std::__ndk1::mutex::unlock()	3	0
std::__ndk1::mutex::~mutex()	2	0

	refs	__cxa_guard_*
#12534 (base)	55	10
this PR	47	10

The link-time libc++ requirement only disappears when every cause reaches zero, so this is one of several prerequisites rather than a self-sufficient win — the remaining causes (operator new/delete[], __cxa_guard_*, std::string, __libcpp_verbose_abort) are tracked in #12533.

Why manual unlock is safe here

The native runtime is built with -fno-exceptions (confirmed in compile_commands.json), so there is no unwind path that std::lock_guard would have covered and a manual unlock would miss. Every return inside a locked region was verified to be immediately preceded by an unlock of that mutex, across all 13 regions.

Verification

• CoreCLR, MonoVM and NativeAOT all build clean. fastdev-assemblies.cc is #if defined(DEBUG) and therefore not built in Release, so it was additionally compiled with -DDEBUG to confirm it is correct.
• Real libc++ references: CoreCLR 55 → 47, NativeAOT 0 — the eight removed are exactly the mutex members listed above, confirmed by diffing the demangled undefined-symbol sets of the two archives.
• __cxa_guard_* undefined references stay at 10. This is the check that matters for correctness of the conversion: pthread_mutex_t is a POD and PTHREAD_MUTEX_INITIALIZER is a constant initializer, so the four statics stay constant-initialized. Had they needed dynamic initialization we would have traded eight mutex symbols for eight guard symbols and gained nothing.
• Only pthread_mutex_lock / pthread_mutex_unlock (both libc) remain as mutex symbols.

[Copilot]

Pull request overview

This PR reduces the CoreCLR host’s dependence on libc++ by replacing std::mutex/std::lock_guard usage with a minimal pthread-backed Mutex/MutexGuard wrapper in the shared native runtime-base headers.

Changes:

• Add Mutex and MutexGuard wrappers around pthread_mutex_t in runtime-base/mutex.hh.
• Update CoreCLR host code paths (assembly store, FastDev assemblies, DSO loader, startup-aware lock) to use the new wrapper instead of <mutex>.
• Update shared timing code to use the new wrapper (affecting other runtime lanes via common sources).

Show a summary per file

File	Description
src/native/common/include/runtime-base/timing.hh	Switch timing sequence locking to Mutex/MutexGuard.
src/native/common/include/runtime-base/mutex.hh	Introduce pthread-backed Mutex and MutexGuard.
src/native/clr/include/runtime-base/startup-aware-lock.hh	Replace std::mutex reference with Mutex.
src/native/clr/include/runtime-base/monodroid-dl.hh	Replace static std::mutex with Mutex for DSO handle write lock.
src/native/clr/include/host/fastdev-assemblies.hh	Replace override directory lock from std::mutex to Mutex.
src/native/clr/include/host/assembly-store.hh	Replace assembly decompress lock from std::mutex to Mutex.
src/native/clr/host/fastdev-assemblies.cc	Replace std::lock_guard usage with MutexGuard.
src/native/clr/host/assembly-store.cc	Replace internal state_lock and std::lock_guard usage with Mutex/MutexGuard.

Review details

• Files reviewed: 8/8 changed files
• Comments generated: 2
• Review effort level: Lite