[clang] [clang][mlibc] Predefine `__mlibc__` for mlibc targets (PR #225654)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

The only mlibc targets which currently have test coverage in clang are "-managarm-mlibc" targets.  Is that the target you're interested in?  If it is, does it make sense to put the new code in ManagarmTargetInfo?

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All new defines should have regression test coverage in clang/test/Preprocessor/init.c .

https://github.com/llvm/llvm-project/pull/225654