kbuild: dummy-tools: adjust to scripts/cc-version.sh

Commit aec6c60a ("kbuild: check the minimum compiler version in
Kconfig") changed how the script detects the compiler version.

Get 'make CROSS_COMPILE=scripts/dummy-tools/' back working again.

Fixes: aec6c60a ("kbuild: check the minimum compiler version in Kconfig")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Miguel Ojeda <ojeda@kernel.org>