[msan][tsan] Refine __fxstat{,at}{,64} condition

In glibc before 2.33, include/sys/stat.h defines fstat/fstat64 to
`__fxstat/__fxstat64` and provides `__fxstat/__fxstat64` in libc_nonshared.a.
The symbols are glibc specific and not needed on other systems.

Reviewed By: vitalybuka, #sanitizers

Differential Revision: https://reviews.llvm.org/D118423