kconfig: shorten the temporary directory name for cc-option

The cc-option macro creates the temporary directory, .tmp_$$$$.
Shell expands '$$' into its process ID. '$$$$' results in repeated PID.

In Makefile, '$$$$' is correct (see TMPOUT in scripts/Malefile.compiler)
because '$$' is an escape sequence of '$'. Make expands '$$$$' into '$$',
then shell expands it into the process ID.

This does not apply to Kconfig because Kconfig requires variable
references to be enclosed by curly braces, like ${variable}.
The '$' that is not followed by '{' loses its effect.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Nicolas Schier <nicolas@fjasle.eu>