kbuild: dummy-tools: avoid tmpdir leak in dummy gcc

When passed -print-file-name=plugin, the dummy gcc script creates a
temporary directory that is never cleaned up. To avoid cluttering
$TMPDIR, instead use a static directory included in the source tree.

Fixes: 76426e23 ("kbuild: add dummy toolchains to enable all cc-option etc. in Kconfig")
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>