[mach-o] refactor x86_64 relocation handling.

This is first step in reworking how mach-o relocations are processed.
The existing KindHandler is going to become a delgate/helper object for
processing architecture specific references.  The KindHandler knows how
to convert mach-o relocations into References and back, as well, as fixing
up the content the relocation is on.

One of the messy things about mach-o relocations is that they sometime
come in pairs, but the pairs still convert to one lld::Reference. So, the
conversion has to detect pairs (arch specific) and change the stride.
[x86] Fix a miscompile in the new shuffle lowering uncovered by
a bootstrap.

I managed to mis-remember how PACKUS worked on x86, and was using undef
for the high bytes instead of zero. The fix is fairly obvious.
Silencing some -Wcast-qual warnings. No functional changes intended.