libgcc: Split FDE search code from PT_GNU_EH_FRAME lookup

libgcc: Split FDE search code from PT_GNU_EH_FRAME lookup

This allows switching to a different implementation for
PT_GNU_EH_FRAME lookup in a subsequent commit.

This moves some of the PT_GNU_EH_FRAME parsing out of the glibc loader
lock that is implied by dl_iterate_phdr.  However, the FDE is already
parsed outside the lock before this change, so this does not introduce
additional crashes in case of a concurrent dlclose.

libgcc/ChangeLog:

	* unwind-dw2-fde-dip.c (struct unw_eh_callback_data): Add hdr.
	Remove func, ret.
	(find_fde_tail): New function.  Split from
	_Unwind_IteratePhdrCallback.  Move the result initialization
	from _Unwind_Find_FDE.
	(_Unwind_Find_FDE): Updated to call find_fde_tail.