[XRay][compiler-rt] Spell REQUIRES properly for x86_64-linux

Until llvm-xray starts running/supporting binaries that are not ELF64 we
only run the FDR tests on x86_64-linux. Previous changes caused the
tests to not actually run on x86_64.

Follow-up on D31454.

llvm-svn: 299050
2 files changed