[libiberty] remove TBAA violation in iterative_hash, improve code-gen

The following removes the TBAA violation present in iterative_hash.
As we eventually LTO that it's important to fix.  This also improves
code generation for the >= 12 bytes loop by using | to compose the
4 byte words as at least GCC 7 and up can recognize that pattern
and perform a 4 byte load while the variant with a + is not
recognized (not on trunk either), I think we have an enhancement bug
for this somewhere.

Given we reliably merge and the bogus "optimized" path might be
only relevant for archs that cannot do misaligned loads efficiently
I've chosen to keep a specialization for aligned accesses.

libiberty/
	* hashtab.c (iterative_hash): Remove TBAA violating handling
	of aligned little-endian case in favor of just keeping the
	aligned case special-cased.  Use | for composing a larger word.