Simplify Makefile by removing OS X targets

Right now the Makefile is cluttered with OS X equivalents of all the x86 targets. We can get rid of all of them and just check UNAME against "Darwin" for the few OS X-specific things we need to do.

We also disable Clang LTO when using BMI2 instructions. For some reason, LLVM cannot find the PEXT instruction when using LTO. I don't know why, but disabling LTO for BMI2 fixes it.