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

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.

Reviewers: kpw, eizan, jfb

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D51277

llvm-svn: 340812

compiler-rt/lib/xray/xray_trampoline_x86_64.S

@@ -19,6 +19,7 @@
 
 
 .macro SAVE_REGISTERS
+	pushfq
 	subq $240, %rsp
 	CFI_DEF_CFA_OFFSET(248)
 	movq %rbp, 232(%rsp)
@@ -69,6 +70,7 @@
 	movq  8(%rsp), %r14
 	movq  0(%rsp), %r15
 	addq	$240, %rsp
+	popfq
 	CFI_DEF_CFA_OFFSET(8)
 .endm