consistent stress test failures on arm64 after recent signal changes

Wed Jun 18 19:04:02 GMT 2025

The "stress tests x86_64 on arm64 Windows" tests "fp" and "memcpy" seem to
be consitently failing since 364226a. The step is allowed to continue on
error because there is some flakiness with tests on arm64 Windows, but
these tests seem to be pretty consistently failing.
It seems like 3 commits were pushed at once, so there's only one job for:
Cygwin: signal: Prevent unexpected crash on frequent SIGSEGV
Cygwin: signal: Revert _cygtls::inside_kernel() change
Cygwin: signal: Enable the signal mask earlier
See for example
https://github.com/cygwin/cygwin/actions/runs/15706999164/job/44256840429#step:6:37