[Clang][LoongArch] Add GPR alias handling without `$` prefix

Currenlty there is a mismatch between LoongArch gcc and clang about
handling register name in inlineasm, i.e. gcc allows both `$`-prefixed
and non-prefiexed names for GPRs while clang only allows `$`-prefixed
one. This patch fixes this mismatch by adding non-prefixed GPR names
in clang.

Take `$r4` for example. With this patch, clang accepts `$r4`, `r4`,
`$a0` and `a0` like what gcc does.

Reviewed By: xen0n

Differential Revision: https://reviews.llvm.org/D136436