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
7 jobs for !17345 with busybox-libgcc-dynamic in 1 minute and 21 seconds (queued for 1 second)
merge request