Remove GOOGLE_PROTOBUF_ARCH_PPC

The macro GOOGLE_PROTOBUF_ARCH_PPC is not used anywhere in the protobuf
source; there is no Power-specific atomics implementation, etc.

Funnily enough, the macro __ppc__ is not actually defined on 32-bit
Power on GCC/Linux, according to the following webpage:

    http://nadeausoftware.com/articles/2012/02/c_c_tip_how_detect_processor_type_using_compiler_predefined_macros#POWER

and verified on a 32-bit Debian sid 'powerpc' chroot:

    (sid_powerpc-dchroot)edmonds@partch:~$ gcc -dM -E - < /dev/null | grep -c __ppc__
    0
    (sid_powerpc-dchroot)edmonds@partch:~$ gcc -dM -E - < /dev/null | grep -c __LP64__
    0