tools: testing: allow importing arch headers in shared.mk

tools: testing: allow importing arch headers in shared.mk

There is an arch/ tree under tools.  This contains some useful stuff, to
make that available, add it to the -I flags.  This requires $(SRCARCH),
which is provided by Makefile.arch, so include that..

There still aren't that many headers so also just smush all of them into
SHARED_DEPS instead of starting to do any header dependency hocus pocus.

Link: https://lkml.kernel.org/r/20250828-b4-vma-no-atomic-h-v2-2-02d146a58ed2@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>