Previously extra linker arguments had different behavior on darwin vs
other unix platforms. On darwin the arguments passed with -Xlinker would
be passed to the linker before the default arguments, where as with the
default unix toolchain they would be passed afterwards.
There isn't really a great option for which order these should be in.
If you want to have a custom rpath that takes precedence over the
default rpaths, you want them to be passed before, but if you want to
negate a default argument you want them to come after.
This change unifies the behavior so at least you always get the same
behavior across platforms.
