[libc++abi] Allow specifying custom Lit config files

This is the libc++abi counterpart of 0c66af970c80.

Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 33c9c10d183371edc95fa936705bef56f55ab611
1 file changed