PowerPC64 ELFv2 ABI 1/6: Code refactoring

PowerPC64 ELFv2 ABI 1/6: Code refactoring

This is the first patch to support the new ELFv2 ABI in glibc.

As preparation, this patch simply refactors some of the powerpc64 assembler
code to move all code related to creating function descriptors (.opd section)
or using function descriptors (function pointer call) into a central place
in sysdep.h.

Note that most locations creating .opd entries were already using macros
in sysdep.h, this patch simply extends this to the remaining places.

No relevant change in generated code expected.