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

This reverts commit e78f95ac.

This adds back the original
commit c603a309 ("x86/mm: Identify the end of the kernel area to be reserved")

Bug: 138463564
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Change-Id: Ia01a9b914beebf392a670f8b8238ebceeb361a1d