[XRay][compiler-rt][x86_64] Preserve flags in x86_64 trampolines. (#89452)

Previously, some xray trampolines would modify condition codes (before
saving/after restoring flags) due to stack alignment instructions, which
use add/sub.

I am not aware of issues that this causes in practice (outside of the
situation described in https://github.com/llvm/llvm-project/pull/89364,
which is only problematic due to a different bug). Nevertheless, it
seems nicer and less error-prone for xray instrumentation to be as
unobstrusive/preserve as much state as possible.