[XRay][compiler-rt] Stash flags as well in x86_64 trampoline

[dberris]

Summary

This change saves and restores the full flags register in x86_64 mode.
This makes running instrumented signal handlers safer, and avoids flags
set during the execution of the event handlers from polluting the
instrumented call's flags state.

lib/xray/xray_trampoline_x86_64.S

 //===----------------------------------------------------------------------===//
 
 #include "../builtins/assembly.h"
 #include "../sanitizer_common/sanitizer_asm.h"
 
 
 
 .macro SAVE_REGISTERS
+  pushfq
   subq $240, %rsp
   CFI_DEF_CFA_OFFSET(248)
   movq %rbp, 232(%rsp)
   movupd  %xmm0, 216(%rsp)
   movupd  %xmm1, 200(%rsp)
   movupd  %xmm2, 184(%rsp)
   movupd  %xmm3, 168(%rsp)
   movupd  %xmm4, 152(%rsp)
.macro RESTORE_REGISTERS
   movq  48(%rsp), %r9
   movq  40(%rsp), %r10
   movq  32(%rsp), %r11
   movq  24(%rsp), %r12
   movq  16(%rsp), %r13
   movq  8(%rsp), %r14
   movq  0(%rsp), %r15
   addq  $240, %rsp
+  popfq
   CFI_DEF_CFA_OFFSET(8)
 .endm
 
 .macro ALIGNED_CALL_RAX
   // Call the logging handler, after aligning the stack to a 16-byte boundary.
   // The approach we're taking here uses additional stack space to stash the
   // stack pointer twice before aligning the pointer to 16-bytes. If the stack
   // was 8-byte aligned, it will become 16-byte aligned -- when restoring the