Allowing to build for riscv32imac-unknown-none-elf

[SimonIT]

I used riscv64-unknown-elf-gcc 10.2.0 for building. With these changes ring builds, but haven't been able to verify if it works, yet.

Please feel free to leave some feedback. Some changes are probably needed in the build.rs, but I'm not sure what they are.

The change in target.h is from #1627

[briansmith]

I am not sure this is correct for all "none" targets, e.g. wasm32v1.

I found when targeting some -none targets in the past that I didn't need to build with -ffreestanding. What happens if we omit it for this target?

[SimonIT]

  cargo:warning=In file included from /mnt/data/Software/Entwicklung/ring/include/ring-core/base.h:26,
  cargo:warning=                 from /mnt/data/Software/Entwicklung/ring/crypto/curve25519/internal.h:18,
  cargo:warning=                 from /mnt/data/Software/Entwicklung/ring/crypto/curve25519/curve25519.c:22:
  cargo:warning=/usr/lib/gcc/riscv64-unknown-elf/10.2.0/include/stdint.h:9:16: fatal error: stdint.h: No such file or directory
  cargo:warning=    9 | # include_next <stdint.h>
  cargo:warning=      |                ^~~~~~~~~~
  cargo:warning=compilation terminated.

The compiler can't find stdint.h.

I'm also not sure what the best filter for that target is

[briansmith]

Please combine this with the if (target.arch == WASM32) || (target.os == "linux" && target.env == "musl" && target.arch != X86_64) below.

If you need to support GCC then you could change the test in that block to also include is_like_gcc or whatever.