[LLDB][LoongArch] Add LoongArch ArchSpec and subtype detection

Define LoongArch architecture subtypes, add the LoongArch ArchSpec bits,
and inspect the ELF header to detect the right subtype based on ELF class.

Here is a simple test:

```
[loongson@linux ~]$ cat hello.c

int main()
{
	printf("Hello, World!\n");
	return 0;
}
[loongson@linux ~]$ clang hello.c -g -o hello
```

Without this patch:

```
[loongson@linux ~]$ llvm-project/llvm/build/bin/lldb hello
(lldb) target create "hello"
error: '/home/loongson/hello' doesn't contain any 'host' platform architectures: unknown
```

With this patch:

```
[loongson@linux ~]$ llvm-project/llvm/build/bin/lldb hello
(lldb) target create "hello"
Current executable set to '/home/loongson/hello' (loongarch64).
(lldb) run
Process 735167 launched: '/home/loongson/hello' (loongarch64)
Hello, World!
Process 735167 exited with status = 0 (0x00000000)
(lldb) quit
[loongson@linux ~]$ llvm-project/llvm/build/bin/llvm-lit llvm-project/lldb/test/Shell/ObjectFile/ELF/loongarch-arch.yaml
llvm-lit: /home/loongson/llvm-project/llvm/utils/lit/lit/llvm/config.py:456: note: using clang: /home/loongson/llvm-project/llvm/build/bin/clang
-- Testing: 1 tests, 1 workers --
PASS: lldb-shell :: ObjectFile/ELF/loongarch-arch.yaml (1 of 1)

Testing Time: 0.09s
  Passed: 1
```

Reviewed By: SixWeining, xen0n, DavidSpickett

Differential Revision: https://reviews.llvm.org/D137057