scripts: gcc-wrapper: Use wrapper to check compiler warnings

Rather than telling gcc to make warnings as errors, and then hunting
down make rules to disable this, leave them as warnings, but invoke
gcc through a wrapper that checks the warnings against a whitelist.

The initial whitelist comes from the warnings in the current build,
all of which have been allowed through Make overrides.

Change-Id: Ifc9952e03d1a5dbcd90819dc7aa0ec7e50328e20
Signed-off-by: David Brown <davidb@codeaurora.org>
(cherry picked from commit d21ced2f0dda815cac69c693cd81e2e75d8f0007)