Remove clang-specific libxml2 check from CMake

clang's own CMake setup handles this as of r210308.

The CMAKE_CROSSCOMPILING special-case will no longer be hard-coded. This was
clearly site-specific to someone's local configuration and should be passed in
at configure time if needed with e.g. -DLIBXML2_LIBRARIES=... (the libxml2
target I tried here doesn't even support liblzma so it's *way* off).

llvm-svn: 210309