Passing a define macro to make/gcc?

I'm working on a powerpc arch target for rust. The libunwind LLVM requires the build system define __ppc__. I tried TARGET_CFLAGS+=-D__ppc__ and TARGET_CXXFLAGS+=-D__ppc__, along with HOST_ versions even though I don't think those are a thing.

I used CMAKE_OPTIONS and CMAKE_HOST_OPTIONS also to no avail..

Anyone have any suggestions on how to do this?