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.
Sponsored by: Netflix
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.
Sponsored by: Netflix
