rfe: allow debug and strip options per subproject

[stsp]

Please allow debug and strip options per
subproject. This is especially important when
some subprojects cross-compile while
others do not. It may be needed that
the cross-compiled code to always be
stripped, for example.

In fact, I wonder why just a couple of
options are allowed to change by subprojects.
I think all should be allowed, except
probably quite a few.

[bonzini]

This is essentially #9398 and is mentioned in the 1.7.0 release notes as upcoming; see #13441 for the draft code.