Fix race in spawn.signal_stress test

The default behavior for a real-time signal is to terminate the process.
If the child we fork sends a SIGRTMIN signal before we've set up the
ScopedSignalHandler for it, then the parent is terminated.

Test: adb shell /data/nativetest64/bionic-unit-tests-static/bionic-unit-tests-static --gtest_filter=spawn.signal_stress --gtest_repeat=1000
Bug: b/77554047
Change-Id: Ib44cbea8ccf506644405d8f426d9ad24e77cfa33
(cherry picked from commit a1bc826c)