[llvm-exegesis] Fix unused variable warning on non-linux platforms

Previous patches moved the save/restore system call register functions
behind a preprocessor check, but neglected to move the constant
expression array used by the functions behind the preprocessor check.
This patch gates the array behind the preprocessor check to fix build
failures caused by -Werror,unused-const-variable on the clang--ppc64-aix
buildbot.