[PowerPC] powerpc64 rtld IFUNC handling code

As PowerPC is moving to clang, we can finally start taking advantage of
IFUNC.

Implement the MD parts of IFUNC handling for rtld.

Currently, it is necessary to look for R_PPC_IRELATIVE in the PLT in
addition to RELA. This is an ABI violation, but LLD9 has some .iplt bugs
that require this as a workaround.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D22789