libgcc: Special-case BFD ld unwind table encodings in find_fde_tail

BFD ld (and the other linkers) only produce one encoding of these
values.  It is not necessary to use the general
read_encoded_value_with_base decoding routine.  This avoids the
data-dependent branches in its implementation.

libgcc/

	* unwind-dw2-fde-dip.c (find_fde_tail): Special-case encoding
	values actually used by BFD ld.