[release-branch.go1.5] runtime: ignore rt_sigaction error if it is for SIGRTMAX

A forward port of https://codereview.appspot.com/124900043/ which somehow
got lost somewhere.

Fixes #13024

Change-Id: Iab128899e65c51d90f6704e3e1b2fc9326e3a1c2
Reviewed-on: https://go-review.googlesource.com/16853Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-on: https://go-review.googlesource.com/16986
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/os1_linux.go

@@ -293,7 +293,8 @@ func setsig(i int32, fn uintptr, restart bool) {
 		fn = funcPC(sigtramp)
 	}
 	sa.sa_handler = fn
-	if rt_sigaction(uintptr(i), &sa, nil, unsafe.Sizeof(sa.sa_mask)) != 0 {
+	// Qemu rejects rt_sigaction of SIGRTMAX (64).
+	if rt_sigaction(uintptr(i), &sa, nil, unsafe.Sizeof(sa.sa_mask)) != 0 && i != 64 {
 		throw("rt_sigaction failure")
 	}
 }