kbuild: dummy-tools: pretend we understand __LONG_DOUBLE_128__

There is a test in powerpc's Kconfig which checks __LONG_DOUBLE_128__
and sets CONFIG_PPC_LONG_DOUBLE_128 if it is understood by the compiler.

We currently don't handle it, so this results in PPC_LONG_DOUBLE_128 not
being in super-config generated by dummy-tools. So take this into
account in the gcc script and preprocess __LONG_DOUBLE_128__ as "1".

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