[XRay][compiler-rt] Support TSC emulation even for x86_64

Summary:
Use TSC emulation in cases where RDTSCP isn't available on the host
running an XRay instrumented binary. We can then fall back into
emulation instead of not even installing XRay's runtime functionality.
We only do this for now in the naive/basic logging implementation, but
should be useful in even FDR mode.

Should fix http://llvm.org/PR32148.

Reviewers: pelikan, rnk, sdardis

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D30677

llvm-svn: 297800