eliminate mutex in fast path of __register_frame

eliminate mutex in fast path of __register_frame

The __register_frame/__deregister_frame functions are used to register
unwinding frames from JITed code in a sorted list. That list itself
is protected by object_mutex, which leads to terrible performance
in multi-threaded code and is somewhat expensive even if single-threaded.
There was already a fast-path that avoided taking the mutex if no
frame was registered at all.

This commit eliminates both the mutex and the sorted list from
the atomic fast path, and replaces it with a btree that uses
optimistic lock coupling during lookup. This allows for fully parallel
unwinding and is essential to scale exception handling to large
core counts.

libgcc/ChangeLog:

	* unwind-dw2-fde.c (release_registered_frames): Cleanup at shutdown.
	(__register_frame_info_table_bases): Use btree in atomic fast path.
	(__deregister_frame_info_bases): Likewise.
	(_Unwind_Find_FDE): Likewise.
	(base_from_object): Make parameter const.
	(classify_object_over_fdes): Add query-only mode.
	(get_pc_range): Compute PC range for lookup.
	* unwind-dw2-fde.h (last_fde): Make parameter const.
	* unwind-dw2-btree.h: New file.