internal: add a wrapper for __libc_start_main for PowerPC(64(le))
This is necessary because the musl and glibc function signatures differ significantly. This code has been written after consulting this with musl upstream and coming to the conclusion that the only place where this can be fixed is gcompat in a hacky way like this, as fixing in either musl or glibc would involve an ABI breakage and generally wouldn't be worth it.
This should allow gcompat to work on all types of PowerPC (32/64/64le). I also successfully bootstrapped the Go compiler using official binaries and gcompat this way.