Rollup merge of #102112 - cuviper:powerpc64-full-relro, r=eholk

Allow full relro on powerpc64-unknown-linux-gnu

This was previously limited to partial relro, citing issues on RHEL6,
but that's no longer a supported platform since #95026. We have long
been enabling full relro in RHEL7's own Rust builds for ppc64, without
trouble, so it should be fine to drop this workaround.

Author: matthiaskrgr