hooks/pre-configure: new hook to generate foo-config wrappers in cross builds.

Script wrappers are created in ${wrksrc}/.xbps/bin and this path is appended
to make the configure scripts detect them.

This avoids adding build deps in hostmakedepends, as well as avoiding modifying
templates to specify the path to the script.

Currently this only creates a wrapper for "icu-config", but can be extended
easily to create more wrappers (freetype, libxml, etc).