rtld: Move powerpc specific code to powerpc files

[andrew]

Summary

There are two variables set by dynamic tags in the powerpc runtime
linker. Now we have a way to split out architecture-specific dynamic
tags use it to handle these.

Obtained from: jhibbits (earlier version)
Sponsored by: Arm Ltd

[andrew]

Untested, but looks correct based on earlier changes from @jhibbits

[kib]

Thank you