HAVE_GPT isn't currently a thing, but HAVE_GELI is. Replace the former
with the latter and remove util.o from the build list (it's picked up
from libsa/libsa32, and that's OK).
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simd / no float stuff is centeralized here. Also centralise
-ffreestanding since it is specified everywhere.
This, along with a change to share/mk/bsd.cpu.mk to include -mno-avx2
in CFLAGS_NO_SIMD should fix building for newer machines (eg with
CPUTYPE=haswell) where clang was generating avx2 instructions.
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