linux-user: signal delivery for i386 guests

[tamnd]

Signal handlers for qemu-i386, for #8.

• i386 gets the old and rt signal frames (setup_frame(), setup_rt_frame(), sigreturn(), rt_sigreturn()) with the FPU state, and the return trampolines on a page where QEMU puts them, as there is no vDSO.
• The compat layer takes rt_sigaction(), sigaction(), sigaltstack(), sigprocmask(), sgetmask(), ssetmask(), sigpending(), sigsuspend() and the 32-bit siginfo and stack_t layouts.
• Guest gets an optional setup_frame hook for targets with the old frame; the others pass None.

Checked on server3 against QEMU 11.1 qemu-i386, at this commit:

• A signal test (old and rt frames, SA_SIGINFO, the alternate stack, fault and trap signals, the old signal calls, sigtimedwait(), waitid()) prints the same output.
• tests/tcg's crash, overflow, procs, prot-none, segfault, sigbus, sigreturn-sigmask, threadcount, vma-pthread and a thread test match. linux-test and signals stop at socket() and timer_create(), which are still ENOSYS on i386.
• clippy, the linux-user and user-common tests, xtask and the Windows check pass.