[Clang][LoongArch] Implement patchable function entry

Similar to D98610 for RISCV.

This is going to be required by the upcoming Linux/LoongArch
[[ https://git.kernel.org/linus/4733f09d88074 | support for dynamic ftrace ]].

Reviewed By: SixWeining, MaskRay

Differential Revision: https://reviews.llvm.org/D141785