[Clang][LoongArch] Define more LoongArch specific built-in macros

Define below macros according to LoongArch toolchain conventions [1].

* `__loongarch_grlen`
* `__loongarch_frlen`
* `__loongarch_lp64`
* `__loongarch_hard_float`
* `__loongarch_soft_float`
* `__loongarch_single_float`
* `__loongarch_double_float`

Note:
1. `__loongarch__` has been defined in earlier patch.
2. `__loongarch_arch` is not defined because I don't know how `TargetInfo` can get the arch name specified by `-march`.
3. `__loongarch_tune` will be defined in future.

[1]: https://loongson.github.io/LoongArch-Documentation/LoongArch-toolchain-conventions-EN.html

Depends on D136146

Differential Revision: https://reviews.llvm.org/D136413