[PPC64] Fix rtld direct exec mode

[luporl]

Summary

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

[jhibbits]

Looks good. Can the same change apply to powerpc?

[luporl]

In D21131#459452, @jhibbits wrote:

Looks good. Can the same change apply to powerpc?

A brief look at powerpc/rtld_start.S code seems to indicate that the same changes apply to powerpc.

But it would be better for someone with 32-bit powerpc hardware to make the change and confirm it works fine.