[BZ 21357] unwind-dw2-fde: Call free() outside of unwind mutex

__deregister_frame_info_bases() calls free() while holding a mutex which
is also used from _Unwind_Find_FDE().  This leads to a deadlock if
AddressSanitizer uses _Unwind_Backtrace() from its free()
implementation.

Checked on mips-linux-gnu and x86_64-linux-gnu.

	[BZ #21357]
	* sysdeps/generic/unwind-dw2-fde.c (__deregister_frame_info_bases):
	Call free() outside of mutex.