[XRay][compiler-rt] Support sled versioning for custom event sleds

Summary:
This change introduces versions to the instrumentation map entries we
emit for XRay instrumentaiton points. The status quo for the version is
currently set to 0 (as emitted by the LLVM back-end), and versions will
count up to 255 (unsigned char).

This change is in preparation for supporting the newer version of the
custom event sleds that will be emitted by the LLVM compiler.

While we're here, we take the opportunity to stash more registers and
align the stack properly in the __xray_CustomEvent trampoline.

Reviewers: kpw, pcc, dblaikie

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D36816

llvm-svn: 311524