kbuild: dummy-tools: fix inverted tests for gcc

There is a test in Kconfig which takes inverted value of a compiler
check:
* config CC_HAS_INT128
        def_bool !$(cc-option,$(m64-flag) -D__SIZEOF_INT128__=0)

This results in CC_HAS_INT128 not being in super-config generated by
dummy-tools. So take this into account in the gcc script.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>