runtime: unblock SIGUSR1 for TestPreemptM
runtime: unblock SIGUSR1 for TestPreemptM

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 <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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