runtime: avoid clobbering C callee-save register in cgoSigtramp

Use R11 (a caller-saved temp register) instead of RBX (a callee-saved
register).

I believe this only affects linux/amd64, since it is the only platform
with a non-trivial cgoSigtramp implementation.

Updates #18328.

Change-Id: I3d35c4512624184d5a8ece653fa09ddf50e079a2
Reviewed-on: https://go-review.googlesource.com/35068Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/sys_linux_amd64.s

@@ -330,9 +330,9 @@ sigtrampnog:
 	// Lock sigprofCallersUse.
 	MOVL	$0, AX
 	MOVL	$1, CX
-	MOVQ	$runtime·sigprofCallersUse(SB), BX
+	MOVQ	$runtime·sigprofCallersUse(SB), R11
 	LOCK
-	CMPXCHGL	CX, 0(BX)
+	CMPXCHGL	CX, 0(R11)
 	JNZ	sigtramp  // Skip stack trace if already locked.
 
 	// Jump to the traceback function in runtime/cgo.