qt: script & webkit depend on BR2_TOOLCHAIN_HAS_SYNC_4

They use __sync_add_and_fetch_4() and other 32-bit atomics, fixes:
http://autobuild.buildroot.net/results/7ac/7acdbc54e26ad2ba4756512614192be0a086f563/



Also remove sparc from supported architectures since it won't grow
atomics support magically.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>