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

[Fangrui Song via cfe-commits]

MaskRay wrote:

[gcc.gnu.org/onlinedocs/gcc/LoongArch-Options.html](https://gcc.gnu.org/onlinedocs/gcc/LoongArch-Options.html) says

‘large (Not implemented yet)’
‘extreme’
This mode does not limit the size of the code segment and data segment. The -mcmodel=extreme option is incompatible with -fplt, and it requires -mexplicit-relocs=always.

Is there a psABI defining 'extreme' or 'large'?

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