mold: dont allow on MacOS

mold: dont allow on MacOS

Mold does not really work on MacOS, when attempting to use it for example
for ubus:
mold: get_self_path is not supportedcollect2: error: ld returned 1 exit status

Which was introduced by [1] so it seems that MacOS is not supported, so
lets make it non selectable when MacOS is the host.

[1] https://github.com/rui314/mold/commit/f9a37e9dd43681758bbc5647ba9e596ec4ea9f33

Link: https://github.com/openwrt/openwrt/pull/18575
(cherry picked from commit 3c65dc367827bc06bd45f7eb375c59192deb0a75)
Link: https://github.com/openwrt/openwrt/pull/20621
Signed-off-by: Robert Marko <robimarko@gmail.com>