Moved detection of jom location to MingwToolChain

As mingw and perhaps other toolchains using gnu makefiles might want
to use jom as well, it should be possible to obtain jom's location
from somewhere else.

Change-Id: I25ac88b51c84a9d19e5e61a4caa53e1b1a5d5fb9
Reviewed-by: hjk <qthjk@ovi.com>