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: jhibbits Differential Revision: https://reviews.freebsd.org/D22855