PowerPC Support

@LouisBrunner Could you recommend how to restore PPC support?

I tried to build `valgrind` for PPC now, and it fails:
```
checking build system type... powerpc-apple-darwin10
checking host system type... powerpc-apple-darwin10
checking for a supported CPU... ok (powerpc)
checking for a 64-bit only build... no
checking for a 32-bit only build... yes
checking for a supported OS... ok (darwin10)
checking for the kernel version... Darwin 10.x (10.8.0) / Mac OS X 10.6 Snow Leopard
checking for a supported CPU/OS combination... no (ppc32-darwin)
configure: error: Valgrind is platform specific. Sorry. Please consider doing a port.
Command failed: cd "/opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_devel_valgrind/valgrind/work/valgrind-3.16.1" && ./configure --prefix=/opt/local --without-mpicc --enable-only32bit --build=powerpc-apple-darwin10
```
(This is under Rosetta, since I am away from my PPC machines now.)

`valgrind` is supposed to work from 10.5 up, and 10.5 supports `ppc32` and `ppc64` (on 10.6 only `ppc32` is supported).

Start with the reported ./configure invocation and its supported CPU/OS combination check for powerpc-apple-darwin10. Determine what is needed for ppc32 and ppc64 on the stated macOS versions; done means the supported configuration is accepted and Valgrind builds successfully.

Written by the indexing model from the issue text.