runtime: make sure BP is saved in nanotime1/walltime1, else frame pointer may not be preserved

nanotime1 and walltime1 do not preserve BP on linux amd64. Previously, this
did not cause a problem, because nanotime/walltime do preserve the BP. But now
with mid-stack inlining, nanotime/walltime are usually inlined, so BP is not
preserved. So, the BP is now wrong in any function after a call to
nanotime()/walltime() on amd64. That means the frame pointer on the stack can
be wrong for any further function call made after the nanotime() call (notably
runtime.main and various GC functions). [386 doesn't use framepointer.]

Fix is to set a frame size of 8 for nanotime1 and walltime1, which means the
standard prolog/epilog that saves/restore BP in the stack frame is added.

I noticed this while investigating issue 16638 (use frame pointers for
runtime.Callers). This change would needed for progress on that issue (which
doesn't have a high priority). Verified that this fix works/is useful for issue
16638.

Change-Id: I19e19ef2c1a517d737a34928baae034f2eb0b2c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/212079
Run-TryBot: Dan Scales <danscales@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/sys_linux_amd64.s

@@ -205,7 +205,8 @@ TEXT runtime·mincore(SB),NOSPLIT,$0-28
 	RET
 
 // func walltime1() (sec int64, nsec int32)
-TEXT runtime·walltime1(SB),NOSPLIT,$0-12
+// non-zero frame-size means bp is saved and restored
+TEXT runtime·walltime1(SB),NOSPLIT,$8-12
 	// We don't know how much stack space the VDSO code will need,
 	// so switch to g0.
 	// In particular, a kernel configured with CONFIG_OPTIMIZE_INLINING=n
@@ -262,7 +263,9 @@ fallback:
 	MOVL	DX, nsec+8(FP)
 	RET
 
-TEXT runtime·nanotime1(SB),NOSPLIT,$0-8
+// func nanotime1() int64
+// non-zero frame-size means bp is saved and restored
+TEXT runtime·nanotime1(SB),NOSPLIT,$8-8
 	// Switch to g0 stack. See comment above in runtime·walltime.
 
 	MOVQ	SP, BP	// Save old SP; BP unchanged by C code.