Any objects with `call` method can be accepted as trap handlers, and
the check for `Proc` type is useless since ruby/ruby@29f5911cf545.

SIGABRT is for abnormal termination so we should handle it to output a bug
report. Specifically, glibc malloc uses it to exit when there is corruption.
For example, the following script produces a double free:

    mem = Fiddle.malloc(10)
    Fiddle.free(mem)
    Fiddle.free(mem)

Before this patch, it just outputs the following and exits:

    free(): double free detected in tcache 2

After this patch, it also outputs a bug report:

    free(): double free detected in tcache 2
    test.rb:11: [BUG] Aborted at 0x000003e8000ab65c

Co-authored-by: Nobuyoshi Nakada <nobu.nakada@gmail.com>

* Fix race condition in signal handler query

* Initialize signal lock dynamically and reset after fork

* Fix signal handler mutex initialization conditions