Switch to a white list of archs wrt. -m32/-m64 in cc cmdlines

[kinke]

According to the gcc 10.1 cmdline docs.

[kubo39]

Maybe it would be better to use -mx32 for x86_64-unknown-linux-gnux32, not sure LDC supports this architecture.
https://gcc.gnu.org/onlinedocs/gcc/x86-Options.html

[kinke]

I doubt we support it, but yeah, it's a valid point - these hardcoded flags might be trouble. Adding them to ldc2.conf would be another option.

[kubo39]

This works fine for me.

$ ./build-ldc/bin/ldc2 --mtriple=riscv64-unknown-none-elf -mcpu=generic-rv64 \
  -mabi=lp64 -mattr=+m,+a,+c --gcc=./gcc/bin/riscv64-unknown-elf-gcc \
  -Xcc=-march=rv64imac -Xcc=-mabi=lp64 -g -betterC hello.d
$ spike pk hello
bbl loaded
Hello, World!

Full description: https://github.com/kubo39/ldc-riscv-hello

[kinke]

Nice. Is the -mabi option for LDC required? It's the first time I see it being used.

[kubo39]

Ahh, it's not needed. Only -Xcc=-mabi=lp64 is required.