Unmask signal before re-raising fatal signal

On Linux, while the signal handler runs, that signal is masked, so in
the rb_bug_for_fatal_signal() code path we didn't get the default signal
action as intended. See signal(7). It worked fine on macOS, though.

Before:

    $ ./miniruby -e 'Process.kill :SIGSEGV, Process.pid'
    <snip>
    Aborted (core dumped)

After:

    $ ./miniruby -e 'Process.kill :SIGSEGV, Process.pid'
    <snip>
    Segmentation fault (core dumped)

Follow-up for 1ac0afab4da "rb_bug_for_fatal_signal: exit with the right
signal".

0 files changed, 0 insertions, 0 deletions