gmp: disable assembly for arc

It has different inline assembly constraints that
are not supported after update to gcc-6.

Fixes:
http://autobuild.buildroot.net/results/c91/c9143eec0834017d57d5352d6fc95b5fae38da00//


and a lot of other failures of gmp-6.1.1 for ARC.

Signed-off-by: Vlad Zakharov <vzakhar@synopsys.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>