kbuild: dummy-tools: adjust to stricter stackprotector check

Commit 3fb0fdb3 ("x86/stackprotector/32: Make the canary into a regular
percpu variable") modified the stackprotector check on 32-bit x86 to check
if gcc supports using %fs as canary. Adjust dummy-tools gcc script to pass
this new test by returning "%fs" rather than "%gs" if it detects
-mstack-protector-guard-reg=fs on command line.

Fixes: 3fb0fdb3

 ("x86/stackprotector/32: Make the canary into a regular percpu variable")
Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>