[Driver] Support -mcmodel= for LoongArch (#72514)

7e425455 rejects unsupported mcmodel options, but normal/medium/extreme
should be supported models for LoongArch according to [gcc
document](https://gcc.gnu.org/onlinedocs/gcc/LoongArch-Options.html).

The mappings among `gcc`, `clang driver`, `clang cc1` and `LLVM (i.e.
llc --code-model=)` are:

|     gcc      |  clang driver |  clang cc1   |    LLVM    |
| -------------  | ------------------ | ----------------- | -------------- |
| normal    |      normal     |   small        |    small    |
| medium  |     medium     |   medium    |    medium |
| extreme  |     extreme     |  large         |     large     |

Link: https://reviews.llvm.org/D150522