Allow generic/custom payload to generate an exe

The datastore value of ARCH has no effect on the array of
architectures the generic/custom payload is compatible with.
This commit forces the payload to update its list of compatible
architectures on generation if the ARCH value is set in the
datastore.

See:

http://dev.metasploit.com/redmine/issues/7755
Merge branch 'fixgenericcustom' of github.com:rsmudge/metasploit-framework into rsmudge-fixgenericcustom