RFC: what should the RV64Y MACHINE_ARCH be?

[brooksdavis]

We're currently using riscv64c following in the footsteps of mips64c and aarch64c (with some debate about wanting it to be morello). With the base ISA being RV64Y, there's a case to be made that MACHINE_ARCH should follow since the target triple is likely to (per discussion with @jrtc27). On the other hand we're using 64c in the contexts such as libcompat and RV64C isn't realistically going to generate a collision.

[bsdjhb]

The generic ELF bits in the kernel are going to probably be elf64c across all arches FWIW, (e.g. "FreeBSD ELF64C" in the ABI column in ps(1) output). That said, the x86-64 vs amd64 thing has been a bit painful, so matching the upstream triple is probably worth doing. I suspect other MACHINE_ARCHes will all be "c".