tools: testing: support EXTRA_CFLAGS in shared.mk

tools: testing: support EXTRA_CFLAGS in shared.mk

This allows the user to set cflags when building tests that use this
shared build infrastructure.

For example, it enables building with -Werror so that patch-check scripts
will fail:

	make -C tools/testing/vma -j EXTRA_CFLAGS=-Werror

Link: https://lkml.kernel.org/r/20250828-b4-vma-no-atomic-h-v2-3-02d146a58ed2@google.com
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Acked-by: Pedro Falcato <pfalcato@suse.de>
Cc: Jann Horn <jannh@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>