[photonlibos]

fix(thread): complete context-switch clobbers and reserve Win64 shadow space (#1720)

• fix(thread): complete clobbers of context switch, align defer stack, skip red zone

switch_context() / switch_context_defer() hide a function call inside an
asm statement, across which other photon threads run and may change any
register or memory. The clobber lists only named GPRs (plus xmm6-15 on
Win64), so the compiler was free to keep values in vector registers, in
memory it had cached, or in the red zone across a switch.

This is what makes GCC 10/11/12 CI jobs crash (SEGFAULT in test-thread,
test-socket, test-pool, test-executor-std, test-exportfs) on part of the
nodes: with -march=native on a CPU model unknown to these versions (most
likely Emerald Rapids, 0xCF), the driver falls back to -march=cooperlake
-mtune=generic, under which GCC keeps the thread pointer behind
return rq.current->error_number in %xmm0 across the switch in
thread_yield_to().

• x86_64 / aarch64: declare all vector registers (and AVX-512 opmask /
  SVE predicate registers when enabled) as clobbered, plus "memory";
  plus "cc" on aarch64.
• x86_64 SysV / Apple arm64: skip the 128-byte red zone before the call,
  as GCC's manual describes for asm containing calls or pushes.
• x86_64: call the defer function with %rsp aligned to 16 bytes instead
  of jumping into it on whatever the switch-point %rsp happens to be.
• Win64: reserve the 32-byte shadow space for the defer function and the
  thread entry; the latter otherwise overlaps the head of struct thread.
• tools/photongdb.py: account for the skipped red zone when recovering
  %rsp of a switched-out thread.

• fix(thread): clarify why volatile registers must be listed as clobbers

The asm statement is not treated as a call, so the ABI's caller-saved
registers are not implied clobbered; nor does the C++ call of the inline
switch_context() help, as it is inlined, or subject to -fipa-ra.

• fix(thread): remove speculative red-zone padding

Restore the original saved-stack layout and photongdb offset. Keep vector/memory/cc clobbers, defer alignment and Win64 shadow space; no current Photon red-zone failure has been demonstrated.

• fix(thread): preserve original defer jump trampolines

• fix(thread): move Win64 shadow-space comment to _photon_thread_stub
  Generated by Backport Auto PR, by cherry-pick related commits.

Please review and decide whether to merge or close this backport PR.

Conflicts

Cherry-pick produced conflicts. Conflict markers are committed as-is; please resolve them manually before merging.

• 26e71f7: thread/thread.cpp

Conflict resolution

Resolved in a14f89f. The only conflict is the Win64 _photon_thread_stub, where release/0.9 still has call _asan_start (main removed it from the Win64 stub in #937). The resolution keeps the sub $32, %rsp shadow-space reservation from #1720 and drops call _asan_start, so the Win64 stub is now identical to main. ASan builds are Linux-only (PHOTON_BUILD_WITH_ASAN fails the CMake configure on other systems), so on Win64 _asan_start was always an empty function. The SysV and aarch64 stubs are unchanged.