icu: Doesn't work on ARC yet

icu depends on __sync_sub_and_fetch and other atomic primitives that
don't exist in the ARC toolchain yet.

[Peter: adjust beecrypt/php comment dependency, don't mention atomic builtins]
Signed-off-by: Mischa Jonker <mjonker@synopsys.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>