main/busybox: disable CONFIG_STATIC_LIBGCC for non-static builds

Previously, busybox linked statically against libgcc unconditionally
with release 1.33.0 this become configurable [1]. Linking dynamically
against libgcc should result in a size decrease.

[1]: https://git.busybox.net/busybox/commit/?id=39646dce329c1b88b1de8ef0c450813765fda202