runtime: handle signal 34 for musl setgid

It has been observed that setgid hangs when using cgo with musl.
This fix ensures that signal 34 gets handled in an appropriate way,
like signal 33 when using glibc.

Fixes #39343

Change-Id: I89565663e2c361f62cbccfe80aaedf290bd58d57
Reviewed-on: https://go-review.googlesource.com/c/go/+/236518


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>