The `setcontext()` implementation of glibc does restore the signal
mask to the target thread during the switch, libucontext [does not][1]
[1]: https://man.archlinux.org/man/libucontext.3.en#CAVEATS
Instead, we just manually enable signals again just before the call
to `setcontext()`.
With this, tests like `tests/core/mutex_canel` or `tests/core/irq` now
pass on `native64` when using libucontext.
Co-authored-by: crasbe <crasbe@gmail.com>
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