multiprocessing.eclass: Fix handling multiple short options (e.g. -kj)

multiprocessing.eclass: Fix handling multiple short options (e.g. -kj)

Improve the regular expressions to handle parameters consisting of
multiple short options (such as -kj). It should be noted that the code
is not perfect but should handle all common (valid) cases; it could e.g.
incorrectly process a short option followed by string arg such as
'-Wfoo.j' although having this in MAKEOPTS is extremely unlikely.