Add risc-v support code for clang -msave-restore

ok kettenis@

Fix RTARCH in our Makefile and lets us access riscv-specific implementations

This brings us fp_mode.c used in softfloat code, and muldi3.S used when
the target ISA doesn't have the Multiply extensio

Fix RTARCH in our Makefile and lets us access riscv-specific implementations

This brings us fp_mode.c used in softfloat code, and muldi3.S used when
the target ISA doesn't have the Multiply extension.

ok kettenis@

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Compile out-of-line helpers for atomic operations which can be enabled
through -moutline-atomics. These are included by default in this updated
version of compiler-rt, we just haven't enabled them y

Compile out-of-line helpers for atomic operations which can be enabled
through -moutline-atomics. These are included by default in this updated
version of compiler-rt, we just haven't enabled them yet. Some ports start
to make use of that option, so it makes sense to provide these helpers.

The helpers would make use of the ARMv8.1 LSE instructions, if we flagged
that the running system supports those. As we do not yet have a mechanism
to show support for LSE, the code will always fall back to regular atomics.

Issue raised by jca@
Tested by phessler@
Input from jsg@
ok kettenis@

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Build libcompiler_rt for riscv64
ok kettenis@

Don't build double-double functions since long double is the same as double
on OpenBSD.

ok gkoehler@

Remove fixtfti.c from ppc-if to unbreak macppc build. I guess kettenis@
was right that this isn't really needed. Now the list is the same as what
we had for the previous compiler-rt version.

fix path for testing if a .S exists

Add build infrastructure for compiler-rt.

ok kettenis@