makefile: Add -Werror support for RHEL

The -Werror flag should be enabled for RHEL. In ARK, however, it is a bit
tricky to enable -Werror as a build bug occasionally leaks through
upstream review and into Linus' tree, and the toolchain is rebased which
causes many new errors to be reported. As a consequence, -Werror cannot
be enabled in ARK but the code to enable it for RHEL can be added to the
top-level makefile.

Additional fixes: Alphabetize Makefile.variables (something I've wanted to
do for a while now) and remove a mention of Makefile.rhkpg.

Signed-off-by: Prarit Bhargava prarit@redhat.com