[libc] support llvm-lit as a test runner

[nickdesaulniers]

llvm-lit is used throughout most of the llvm-project. Users new to llvm-libc from other parts of llvm may be surprised that llvm-lit is not used for llvm-libc. We should support running unit tests via llvm-lit.

See also discussion from #117253.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

llvm-lit is used throughout most of the llvm-project. Users new to llvm-libc from other parts of llvm may be surprised that llvm-lit is not used for llvm-libc. We should support running unit tests via llvm-lit.

See also discussion from #117253.