[PATCH 0/1] MR5227: ntdll: Respect red zone in usr1_handler() on x64.

[Paul Gofman (＠gofman)]

Fixes a regression introduced by efd3d31082645fab83c6a4994705654edafa9163 ("ntdll: Don't copy xstate from / to syscall frame in usr1_handler().").

SysV x64 ABI assumes 128 byte red zone below stack pointer (for optimizing leaf functions by using the space below rsp saving some stack allocation instructions). I stomped on that red zone is used for real in win32u Unix lib leaf functions causing random crashes inside win32u syscalls when sigusr1 clobbers that zone.