RISCV: Backport some upstream changes to support riscv64

[Ast-x64]

Mainly based on dlang#3846 and dlang#3854.

[kinke]

Hmm, if the upstream PRs had targeted stable, it would have made it into v1.30 final automatically. - I'm not too keen on cherry-picking non-trivial v2.101 stuff for v1.30 and cause potential merge issues for v2.101 later on.

[Ast-x64]

Hmm, if the upstream PRs had targeted stable, it would have made it into v1.30 final automatically. - I'm not too keen on cherry-picking non-trivial v2.101 stuff for v1.30 and cause potential merge issues for v2.101 later on.

Thanks for your reply. I understand your merging consideration. Currently I'm backporting this in order to port LDC to riscv64gc platform, which depends on these modifications. And codes in the second commit here seem to be LDC owned codes (which do not affect upstream dlang/druntime).

[kinke]

Thanks, I've rebased this locally, after which the 1st commit resulted in a no-op as expected; the 2nd commit is in branch ldc now.