[lldb] Support both RISCV-32 and RISCV-64 in GetRegisterInfo (#176472)

`GetRegisterInfo` hardcodes to use `RegisterInfoPOSIX_riscv64` instead
of checking the triple to determine whether to use
`RegisterInfoPOSIX_riscv64` or `RegisterInfoPOSIX_riscv32`.

Someone put up a [PR](https://github.com/llvm/llvm-project/pull/175262)
for this, but seems to have removed their account and the associated PR
with it.

Fixes #175092

GitOrigin-RevId: a84d74355e741ba5b9fd68eff24216d086de09f2