either aborts or exits, but never returns. Tag it as a non-returning
function rather than supply a bogus return(0) at the end of main.
CID: 1382885
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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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