TestPreemptM tests signal delivery using SIGUSR1, but (for unknown
reasons) SIGUSR1 is blocked by default on android/arm and
android/arm64, causing the test to fail.

This fixes the test by ensuring that SIGUSR1 is unblocked for this
test.

Updates #10958, #24543.

Change-Id: I9f81fbab53f96c74622aabcb6f5276f79e2b6d33
Reviewed-on: https://go-review.googlesource.com/c/go/+/203957
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>