Prefer lualatex, xelatex, pdflatex, in that order. #12804

Bugs in pdflatex (#12804) are preventing the guide from landing (https://github.com/rust-lang/rust/pull/16657). This solves the immediate problem by changing the build system to prefer lualatex, xelatex to pdflatex (which is apparently obsolete). Because the xelatex on the snapshot bot seems to completely ignore the `-output-directory` option, I also had to frob the makefiles a bit for that case.
(typo)