Propagate "-arch x86_64h" setting to the linker. <rdar://problem/15711488>

This is a follow-up to r194907, which added a new -arch setting to make it
easier to specify AVX2 targets. The "-arch x86_64h" option needs to be passed
on to the linker, but it was getting canonicalized to x86_64 by the code
in getArchTypeForDarwinArchName.