Move kernel includes and libsa includes together at the top of defs.mk
Move all machine specific defines from Makefile.inc to their friends
in defs.mk.
Add comments and remove now useless junk after the move.
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riscv doesn't have -msoft-float. For the moment, just don't add
anything. There's no /boot/loader or other bootstrap contained in the
tree for riscv*. However, with real hardware coming next year, there
are plans for one, so keep building at least a minimal libsa and
ficl to prevent bitrot.
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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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