kbuild: sync with Linux 4.3

Update some build scripts to match Linux 4.3.  Commit-based syncing was
done so as to not break U-boot specific changes.  This continues the
syncing process started previously in this series.

The commits imported from Linux (some with adjustments) are:

[1] commit 26ea6bb1fef06c686be771903ecab0518af5c2de
    Author: Behan Webster <behanw@converseincode.com>
    kbuild, LLVMLinux: Supress warnings unless W=1-3

[For all of the Kbuild changes within Linux]
Signed-off-by: Behan Webster <behanw@converseincode.com>
Signed-off-by: Jan-Simon Möller <dl9pf@gmx.de>
Signed-off-by: Mark Charlebois <charlebm@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>

[For applying within U-Boot]
Signed-off-by: Tom Rini <trini@konsulko.com>