ensure that compile-flags arguments are the last in ui tests

ensure that compile-flags arguments are the last in ui tests

Before this commit, compiletest would add `-L path/to/aux` at the end of
the rustc flags, even after the custom ones set with the compile-flags
header comment. This made it impossible to check how rustc would behave
when a flag requiring an argument was passed without the argument,
because the argument would become `-L`.

This PR fixes that by adding the `-L path/to/aux` before the arguments
defined in compile-flags, at least for UI tests. Other test suites might
either be fixed as well by this change, or still present the old
behavior.