crimson/common: explicitly reraise handled signal in FatalSignal.

crimson/common: explicitly reraise handled signal in FatalSignal.

Over the current approach where we just reset the handler to
default and allow CPU to re-execute the segfaulting instruction,
the explicit `::reraise()` is:
1. immune to a race condition if muliple threads run into
   troubles the same time;
2. easier to understand and similar to the classic OSD.

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>