bfd: optimize bfd_elf_hash

bfd: optimize bfd_elf_hash

The bfd_elf_hash loop is taken straight from the sysV document, but it
is poorly optimized. This refactoring removes about 5 x86 insns from
the 15 insn loop.

1) The if (..) is meaningless -- we're xoring with that value, and of
course xor 0 is a nop. On x86 (at least) we actually compute the xor'd
value and then cmov.  Removing the if test removes the cmov.

2) The 'h ^ g' to clear the top 4 bits is not needed, as those 4 bits
will be shifted out in the next iteration.  All we need to do is sink
a mask of those 4 bits out of the loop.

3) anding with 0xf0 after shifting by 24 bits can allow betterin
encoding on RISC ISAs than masking with '0xf0 << 24' before shifting.
RISC ISAs often require materializing larger constants.

	bfd/
	* elf.c (bfd_elf_hash): Refactor to optimize loop.
	(bfd_elf_gnu_hash): Refactor to use 32-bit type.