Fix building qt*-*.9999

Without this fix, it is impossible to build qtcore-4.8.9999 (and very likely all other 4.8.9999 and .9999 Qt components), as this makes $WORKDIR being

    /var/tmp/portage/dev-qt/qtcore-4.8.9999/work/qt-everywhere-opensource-src-4.8.9999

instead of

    /var/tmp/portage/dev-qt/qtcore-4.8.9999/work/qtcore-4.8.9999

By resetting `$MY_P` to `$P` for .9999 ebuilds, everything should work again.

Please review first - I'm not sure how Qt 5.x ebuilds might be affected here.