PowerPC: Support ELFv2 ABI

The ELFv2 ABI does not use function descriptors.  This change has been
tested on Power Mac G5, IBM POWER6, and Raptor Talos II, all using the
musl libc on Linux kernel 4.14.
1 job for master in 1 minute and 9 seconds (queued for 1 second)
Status Job ID Name Coverage
passed #91915
debian sid