Add Modula-2 set operations to PowerPC ncg.

Add Modula-2 set operations to PowerPC ncg.

This provides and, ior, xor, com, zer, set, cms when defined($1) and
ior, set when !defined($1).  I don't provide the other operations
!defined($1) because our Modula-2 compiler hasn't used them.

I wrote a Modula-2 example in
https://gist.github.com/kernigh/add79662bb3c63ffb7c46d01dc8ae788

Put a dummy comment in mach/powerpc/libem/build.lua so git checkout
will touch that file.  Without the touch, the build system doesn't see
the new *.s files.