Implement a workaround for clang + PARAVIRT failure.

Make sure that CC is passed to kbuild when we build the kernel.

We must disable CONFIG_PARAVIRT due to an upstream bug. Apply the
workaround unconditionally, as PARAVIRT does nothing on UML.

Bug: 79694505
Test: CC=clang build/build_test.sh
Change-Id: Iaa4083bbb6505a04d4140ed8ef0c84ad0358e061
Signed-off-by: Alistair Strachan <astrachan@google.com>