lit: Make config.llvm_libxml2_enabled an int, not a string

@LLVM_LIBXML2_ENABLED@ will be replaced with 0 or 1. Putting quotes
around that is unnecessary and just makes it harder to use the value.
This matches what have_zlib does below.

This also puts the flag together with the feature-related ones instead
of the path-related flags.

llvm-svn: 327964