mcg broken by changes to PowerPC libem

[kernigh]

In my pull request #59, @davidgiven reports, "the libem ABI changes break mcg."

I change the ABI in PowerPC libem because I rename some procedures and move some parameters to different locations. I change ncg to use the new ABI, but I don't change mcg. This breaks mcg, because ncg and mcg share the same libem. It remains broken until someone modifies mcg and/or libem to fit together.

We currently don't build mcg by default.

• plat/linuxppc/build-tools.lua commented out the dependency of +tools on +mcg, so it doesn't build mcg for linuxppc.
• The top build.lua commented out qemuppc, so it doesn't enter plat/qemuppc and doesn't build mcg for qemuppc.
• We never built mcg for osxppc.

[kernigh]

In pull request #73, I got mcg with qemuppc to pass tests/plat. I believe that mcg works with the tested part of PowerPC libem. There are probably still problems between mcg and the floating-point code in libem, because we have almost no tests for floating point.