[Clang][LoongArch] Handle -march/-m{single,double,soft}-float/-mfpu options

This patch adds options -march, -msingle-float, -mdouble-float,
-msoft-float and -mfpu for LoongArch.

Clang options `msingle_float` and `mdouble_float` are moved from
`m_mips_Features_Group` to `m_Group` because now more than targets use
them.

Reference:
https://github.com/loongson/LoongArch-Documentation/blob/main/docs/LoongArch-toolchain-conventions-EN.adoc

TODO: add -mtune.

Differential Revision: https://reviews.llvm.org/D136146