runtime: use correct psABI SP alignment before calling libc mmap

Fixes #14384.

Change-Id: Ib025cf2d20754b4c2db52f0a8a4717fd303371d6
Reviewed-on: https://go-review.googlesource.com/19660
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-on: https://go-review.googlesource.com/22037

src/runtime/sys_linux_amd64.s

@@ -258,7 +258,7 @@ TEXT runtime·sysMmap(SB),NOSPLIT,$0
 
 // Call the function stored in _cgo_mmap using the GCC calling convention.
 // This must be called on the system stack.
-TEXT runtime·callCgoMmap(SB),NOSPLIT,$0
+TEXT runtime·callCgoMmap(SB),NOSPLIT,$16
 	MOVQ	addr+0(FP), DI
 	MOVQ	n+8(FP), SI
 	MOVL	prot+16(FP), DX
@@ -266,7 +266,11 @@ TEXT runtime·callCgoMmap(SB),NOSPLIT,$0
 	MOVL	fd+24(FP), R8
 	MOVL	off+28(FP), R9
 	MOVQ	_cgo_mmap(SB), AX
+	MOVQ	SP, BX
+	ANDQ	$~15, SP	// alignment as per amd64 psABI
+	MOVQ	BX, 0(SP)
 	CALL	AX
+	MOVQ	0(SP), SP
 	MOVQ	AX, ret+32(FP)
 	RET