[llvm] Add managarm support (#138854)
This PR is part of a series to upstream managarm support, as laid out in
the
[RFC](https://discourse.llvm.org/t/rfc-new-proposed-managarm-support-for-llvm-and-clang-87845/85884/1).
This PR is a follow-up to #87845.
The intention here is to add the managarm target to the LLVM support
lib, in preparation for a follow-up that would add the missing clang
driver bits.
