LoongArch: extable: Also recognize ABI names of registers

When the kernel is compiled with LLVM, the register names being handled
during exception fixup building are ABI names instead of bare $rNN
style. Add mapping for the ABI names for LLVM compatibility.

Signed-off-by: WANG Rui <wangrui@loongson.cn>
Signed-off-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>