Revert r327964 "lit: Make config.llvm_libxml2_enabled an int, not a string"

It seems @LLVM_LIBXML2_ENABLED@ doesn't always get set to 0 when not available,
but to nothing, which broke parsing of lit.site.cfg.py.

> @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: 327966