[PPC64] Fix rtld direct exec mode

Instead of restoring the saved values of argc, argv and envp,
these must be loaded from the stack that _rtld() modifies.

This fixes rtld direct exec mode.
E.g.: /libexec/ld-elf.so.1 /bin/ls

Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D21131