kbuild: drop CONFIG_USE_PRIVATE_LIBGCC=path/to/libgcc syntax

Now CONFIG_USE_PRIVATE_LIBGCC is only used as a boolean macro.
Remove CONFIG_USE_PRIVATE_LIBGCC=path/to/libgcc syntax.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Tested-by: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>