This fixes an endless loop in the absl Arm stack unwinder where encountering a
second signal return trampoline (as one has in nested signal frames), would restart
unwinding at the outermost signal, resulting in an endless loop.

This does not change any behavior in the non-nested signal case, so I believe
it is safe for any stack that hasn't encountered this bug already.

I would love to test this beyond the absl unwinding test cases and the
fingerprint_test included here, but I'm at a loss for other test cases.

PiperOrigin-RevId: 539113007
Change-Id: I10037f9fa77b45cc4db61f89b9c6380ec3529113