runtime: ignore GNU/Linux sigaction errors for signals 32 and 33

This avoids problems when running under QEMU. It seems that at least
some QEMU versions turn the sigaction implementation into a call to
the C library sigaction function. The C library function will reject
attempts to set the signal handler for signals 32 and 33. Ignore
errors in that case.

Change-Id: Id443a9a32f6fb0ceef5c59a398e7ede30bf71646
Reviewed-on: https://go-review.googlesource.com/125955


Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>