[compiler-rt] Mark $t* as clobbered for Linux/LoongArch syscalls

Linux/LoongArch doesn't preserve temporary registers across syscalls,
so we have to explicitly mark them as clobbered to avoid trashing local variables.

Reviewed By: xry111, xen0n, tangyouling, SixWeining

Differential Revision: https://reviews.llvm.org/D137396