[compiler-rt][XRay] Stash xmm registers in the trampolines

[dberris]

Summary

We now stash and restore the xmm registers in the trampolines so that
log handlers don't need to worry about clobbering these registers.

In response to comments in D21612.

[dberris]

• Fixed the wrong math on stack adjustment

[rSerge]

lib/xray/xray_trampoline_x86.S

92–97 ↗

(On Diff #66432)

Wouldn't it suffice to save only xmm0 and xmm1 here? As I understood, no calling convention uses registers beyond xmm1 for return values: http://www.agner.org/optimize/calling_conventions.pdf

[dberris]

Only save xmm0 and xmm1 on the exit trampoline

lib/xray/xray_trampoline_x86.S

92–97 ↗

(On Diff #66432)

It should be -- thanks for the link!

[rnk]

lgtm