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.

No functional change.
More readable trapped rook condition

Prefer

file_of(s) < file_of(ksq)

to the inidrect

file_of(ksq) < FILE_E

To evaluate if semiopen side to check is the left side.

Also other small touches while there.

No functional change.