Revert "Revert "x86/mm: Identify the end of the kernel area to be reserved""

This reverts commit 307cefb8 which was a
revert of c603a309 from upstream.

Yes, this breaks the build when using the gold linker for x86, but
upstream has now blacklisted that linker so trying to work around this
by reverting random patches is not going to be a viable solution.

So revert this commit, and deal with the fallout in the build system by
fixing the build infra properly.

Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: Iae2155c02cd5d6a7981e0151e22f024dc6d4f86b