[libcxx] Always convert 'use_system_cxx_lib' to an absolute path
[libcxx] Always convert 'use_system_cxx_lib' to an absolute path

Otherwise, some tests would fail when a relative path was passed,
because they'd use the relative path from a different directory
than the current working directory.

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@348525 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx