[llvm] [llvm][MC] Fix missing SubArch information when calling lookupTarget (PR #69902)

[via llvm-commits]

huaatian wrote:

> If it does in fact work correctly then you need a test to demonstrate it.

I added a unit test case for the mismatch between Arch and SubArch, as well as a test case for the expected scenario. In the test case, we expect to preserve the SubArch information in the target triple.

https://github.com/llvm/llvm-project/pull/69902