runtime: Remove lookup-asm-powerpc-macho.S

The newest compilers for macOS/PowerPC, Apple GCC 4.0.1 and Apple GCC
4.2.1, both fail to compile any code that contains fast enumeration when
using -fgnu-runtime (both crash with a bus error). This means that using
the ObjFW runtime on macOS/PowerPC is no longer possible since ObjFW
started requiring ObjC 2.

macOS/PowerPC with the Apple runtime is still fully supported.