runtime: fix misaligned SP for libfuzzer entry

libfuzzer is written in C and so requires by the C abi that SP be
aligned correctly mod 16. Normally CALLs need to have SP aligned to 0
mod 16, but because we're simulating a CALL (which pushes a return
address) with a JMP (which doesn't), we need to align to 8 mod 16
before JMPing.

This is not causing any current problems that I know of. All the
functions called from this callsite that I checked don't rely on
correct alignment.  So this CL is just futureproofing.

Update #49075

Change-Id: I13fcbe9aaf2853056a6d44dc3aa64b7db689e144
Reviewed-on: https://go-review.googlesource.com/c/go/+/494117
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>

Author: randall77

src/runtime/libfuzzer_amd64.s

@@ -93,6 +93,7 @@ TEXT	runtime·libfuzzerCallTraceIntCmp(SB), NOSPLIT, $0-32
 	MOVQ	(g_sched+gobuf_sp)(R10), SP
 call:
 	ANDQ	$~15, SP	// alignment for gcc ABI
+	SUBQ	$8, SP
 	// Load the address of the end of the function and push it into the stack.
 	// This address will be jumped to after executing the return instruction
 	// from the return sled. There we reset the stack pointer and return.