[llvm-exegesis] Adjust GLIBC_INITS_RSEQ condition

Commit 9f80831f introduced `#include <sys/rseq.h>`,
but RSEQ_SIG is only defined by some glibc ports (aarch64,arm,mips,powerpc,s390,x86),
causing other hosts (e.g., riscv64, loongarch64) to fail to build.

Reviewed By: aidengrossman, xen0n

Differential Revision: https://reviews.llvm.org/D153938