xcompile: Apply -Wextra with temporary exceptions to GCC

xcompile: Apply -Wextra with temporary exceptions to GCC

In order to detect more issues in our code, make GCC more picky by
enabling -Wextra. Disable a couple of warnings turned on by -Wextra
temporarily in order to keep everything compiling and working for now.
The warnings may be enabled step by step later.

Since xcompiles applies to coreboot and libpayload, add Wextra here
instead of the top-level Makefile.mk.

Change-Id: I60915cb66581dc2c9b6807335fd0e214b45e76d6
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/83347
Reviewed-by: Martin L Roth <gaumless@gmail.com>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>