fix assert in __deregister_frame_info_bases

fix assert in __deregister_frame_info_bases

When using the atomic fast path deregistering can fail during
program shutdown if the lookup structures are already destroyed.
The assert in __deregister_frame_info_bases takes that into
account. In the non-fast-path case however is not aware of
program shutdown, which caused a compiler error on such platforms.
We fix that by introducing a constant for in_shutdown in
non-fast-path builds.
We also drop the destructor priority, as it is not supported on
all platforms and we no longer rely upon the priority anyway.

libgcc/ChangeLog:
	* unwind-dw2-fde.c: Introduce a constant for in_shutdown
	for the non-fast-path case. Drop destructor priority.