Add support for powerpc

[barracuda156]

I have confirmed that the executable works.

[gyurix]

Idea is useful, but patch does not match existing mac target conventions.

Current files use gcc/g++, warn_gcc.mak, and O=b/g, while existing mac targets in-tree use clang + warn_clang_mac.mak + O=b/m_$(PLATFORM). This looks copy-pasted from non-mac configs instead of aligned with current mac build setup.

Merge readiness: 4/10. Before merge: match existing mac toolchain conventions, show successful builds for both ppc and ppc64, and explain expected ASM/toolchain support.

[barracuda156]

Configs are inconsistent here (gcc conventions contradict mac conventions), but since the choice is non-functional, I used a simpler one. I will change that to match clang config in that part.

[barracuda156]

@gyurix I have updated the PR to use m_$(PLATFORM), matching clang configs.

show successful builds for both ppc and ppc64

You will need either native hardware or a set-up with 10.5/10.6 with Rosetta to run these, but binaries are available here:
http://macos-powerpc.org/packages/7zip (10.6 ppc)
http://macos-powerpc.org/packages_ppc64/7zip (10.5 ppc64)

and explain expected ASM/toolchain support.

powerpc*-apple-darwin is supported only in gcc, so that is the requirement. Since generic code is used, there is no specific ASM requirement, Apple cctools (or xtools) will work.

MacPorts uses the latest gcc which is set as the primary one (MacPorts-wide), since portfile excludes old compilers:
https://github.com/macports/macports-ports/blob/de3f1670a63f30fb07443c7bb16f10ddbd3d7106/archivers/7zip/Portfile#L69-L71
Currently, that will be gcc15 is upstream MacPorts (or gcc14 in my fork at the moment).