[Clang][LoongArch] Add initial LoongArch target and driver support

With the initial support added, clang can compile `helloworld` C
to executable file for loongarch64. For example:

```
$ cat hello.c
int main() {
  printf("Hello, world!\n");
  return 0;
}
$ clang --target=loongarch64-unknown-linux-gnu --gcc-toolchain=xxx --sysroot=xxx hello.c
```

The output a.out can run within qemu or native machine. For example:

```
$ file ./a.out
./a.out: ELF 64-bit LSB pie executable, LoongArch, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-loongarch-lp64d.so.1, for GNU/Linux 5.19.0, with debug_info, not stripped
$ ./a.out
Hello, world!
```

Currently gcc toolchain and sysroot can be found here:
https://github.com/loongson/build-tools/releases/download/2022.08.11/loongarch64-clfs-5.1-cross-tools-gcc-glibc.tar.xz

Reference: https://github.com/loongson/LoongArch-Documentation
The last commit hash (main branch) is:
99016636af64d02dee05e39974d4c1e55875c45b

Note loongarch32 is not fully tested because there is no reference
gcc toolchain yet.

Differential Revision: https://reviews.llvm.org/D130255