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

[Zhao Jiazhong via cfe-commits]

zjiaz wrote:

FWIW, `mcmodel=medium` option is used in [chromium's build config file](https://source.chromium.org/chromium/chromium/src/+/main:build/config/compiler/BUILD.gn;l=1451?q=mcmodel&ss=chromium%2Fchromium%2Fsrc:build%2F), and now chromium's llvm toolchain has been affected by this issue:
> clang++: error: unsupported argument 'medium' to option '-mcmodel=' for target 'loongarch64-unknown-linux-gnu'

https://github.com/llvm/llvm-project/pull/72514