Fix signal unsafe call inside a signal

It can easily happen that the signal handler function
`handle_fatal_signal` uses various signal unsafe functions.
The problematic functions are `_` and `strsignal` which
can be pre-computed after the `setlocale` call is done.

Unfortunately when compiled with --disable-libbacktrace a
different code path is used, that calls the glibc function
`backtrace` which calls `malloc` and `free` and is therefore
also signal unsafe, that is probably unfixable, so there
is no attempt to fix anything in this code path.

Approved-By: Andrew Burgess <aburgess@redhat.com>

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31713#c9