glibc: Grab the right linux headers when build != host

In #28519 / 791ce593 I made linux
headers be intended to be used from the stage stage, as it would be if
it were a library containing headers and code. I forgot to update glibc,
however, so it was incorrectly using headers for the build platform, not
host platform.

This fixes that, basically reverting a small portion of changes I made a
few months ago in 25edc476 and its
parent.

No native hashes are changed.