runtime: fix traceback from goexit1

runtime: fix traceback from goexit1

We used to not call traceback from goexit1.
But now tracer does it and crashes on amd64p32:

runtime: unexpected return pc for runtime.getg called from 0x108a4240
goroutine 18 [runnable, locked to thread]:
runtime.traceGoEnd()
    src/runtime/trace.go:758 fp=0x10818fe0 sp=0x10818fdc
runtime.goexit1()
    src/runtime/proc1.go:1540 +0x20 fp=0x10818fe8 sp=0x10818fe0
runtime.getg(0x0)
    src/runtime/asm_386.s:2414 fp=0x10818fec sp=0x10818fe8
created by runtime/pprof_test.TestTraceStress
    src/runtime/pprof/trace_test.go:123 +0x500

Return PC from goexit1 points right after goexit (+0x6).
It happens to work most of the time somehow.

This change fixes traceback from goexit1 by adding an additional NOP to goexit.

Fixes #9931

Change-Id: Ied25240a181b0a2d7bc98127b3ed9068e9a1a13e
Reviewed-on: https://go-review.googlesource.com/5460
Reviewed-by: Russ Cox <rsc@golang.org>