[native] Remove the last libc++ dependencies from the CoreCLR host

[simonrozsival]

Part of #12533 (drop libc++ from the CoreCLR host). Stacked on top of #12570.

Context: #12533

assembly-store.cc was the last file in the CoreCLR host referencing
libc++. With it converted the host is at zero undefined libc++
references, which means the linker now pulls nothing out of
libc++_static.a at all.

The decompressed-assembly cache accounted for all of it:

• cache_dir was a std::string built by repeated append. It is now
  a single snprintf into a stack buffer which the store ID is
  appended to in place, then strduped for the lifetime of the
  process. Both directory levels are still created and validated in
  turn.

• build_path returned a std::string per call. It becomes
  format_cache_path, formatting into a caller-supplied buffer.

• WriteRequest held a std::string path and a
  std::unique_ptr<uint8_t[]> data. The path is gone entirely -
  cache_dir is immutable once the cache is enabled, so the writer
  thread rebuilds the path from descriptor_index alone - and the
  payload now lives immediately after the structure, making a request
  and its bytes a single allocation instead of two.

• std::deque<WriteRequest> becomes an intrusive FIFO threaded
  through WriteRequest::next. The queue is only ever pushed at the
  tail and popped at the head, so a singly linked list is a complete
  replacement; it also removes the per-request node allocation the
  deque made on top of the payload allocation.

• tracking was a std::unique_ptr<uint8_t*[]> and
  assembly_store_names a new std::string_view[]. Both are now
  calloc/free.

• The two std::to_string calls and the .tmp. scan in
  remove_stale_temp_files become snprintf and strstr.

Both snprintf-formatted paths are bounded by Util::LocalPathBufferSize
and checked for truncation, where the std::string versions grew without
limit. A path that long could not be opened anyway, and the failure is
logged and disables the cache rather than being silently ignored.

Behaviour is otherwise unchanged. Allocation failure still disables the
cache instead of taking the process down, except for
assembly_store_names, which is required for correct assembly lookup and
where new[] would previously have aborted anyway - exceptions are
disabled, so its failure called std::terminate.

Results

Undefined libc++ references in the CoreCLR host, Release, arm64:

object	before	after
assembly-store.cc.o	12	0
host total	12	0

	before	after	delta
libnet-android.release.so	520,112	203,776	-316,336 B

The size drop is far larger than the source change because reaching zero
references means the linker stops pulling members out of
libc++_static.a entirely.

Verification

• CoreCLR, MonoVM and NativeAOT all build clean.
• llvm-nm --undefined-only over every CoreCLR host object reports 0
  libc++ references; the 12 present before this change are gone.
• The resulting .so still exports JNI_OnLoad and the
  Java_mono_android_Runtime_* entry points, and its DT_NEEDED list
  is unchanged.
• Relinking with -nostdlib++ in place of -static-libstdc++
  succeeds, confirming that the host no longer needs libc++ at link
  time. Actually dropping the flag is left to a follow-up.

Only android-arm64 was built locally; the other ABIs rely on CI. The
cache itself is exercised by the existing assembly store tests.

[Copilot]

Copilot review overview

Review tier: Lite
Findings: 1

New issues introduced by this change (1)

Severity	Finding
	src/​native/​clr/​host/​assembly-store.cc — ⚠️ Path truncation in format_cache_path() currently fails silently. If cache_dir is near…

What changed in this PR

This PR continues the CoreCLR-host libc++ removal work by refactoring assembly-store.cc to eliminate remaining std::string/std::deque/std::unique_ptr usage in the decompressed-assembly cache path and write-queue implementation, bringing the host to zero undefined libc++ references.

Changes:

• Replaces std::deque<WriteRequest> with an intrusive FIFO (WriteRequest::next) and moves request payload inline (single allocation per request).
• Converts cache path construction from std::string building to bounded snprintf into Util::LocalPathBufferSize stack buffers.
• Replaces new[]-allocated tracking/name tables with calloc/free, and makes cache_dir a strdup-owned immutable C string.

File	Description
src/​native/​clr/​host/​assembly-store.cc	Removes remaining libc++-driven types from the decompressed-assembly cache (paths, write queue, tracking/name tables) using C allocation + fixed buffers.