[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.





git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@211921 91177308-0d34-0410-b5e6-96231b3b80d8