crash: do not use fiber() macro in crash_collect()

Currently, tarantool uses fiber() macro in crash_collect()
to collect backtraces, which is redundant and leads to
NULL dereferencing if crash signal callback is executed
on thread with no initialized cord.

The patch makes it possible not to use fiber module for
collecting backtraces and gets rid of fiber() macro
in crash_collect().

NO_CHANGELOG=internal
NO_TEST=internal
NO_DOC=internal