[utils] Add libc++ and libc++abi config to llvm-lit
[utils] Add libc++ and libc++abi config to llvm-lit

This allows us to use bin/llvm-lit to run individual libc++ and
libc++abi tests without having to explicitly specify the site config
paths, similar to other projects.

Differential Revision: https://reviews.llvm.org/D28733

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@292203 91177308-0d34-0410-b5e6-96231b3b80d8

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