[llvm-exegesis] Don't try to use SYS_rseq if it's not defined.

When compiling against recent glibc (>= 2.35) but old kernel headers (< 4.18), `SYS_rseq` is not defined and thus llvm-exegesis fails to build. So also check that `SYS_rseq` is defined before trying to use it.

Fixes https://github.com/llvm/llvm-project/issues/64456

Reviewed By: MaskRay, gchatelet

Differential Revision: https://reviews.llvm.org/D157189