gmp: Enable ASM for ARC again

This commit reverts cdf63517 "gmp: disable assembly for arc"
as in GMP v6.1.2 there's already a proper fix for ASM constraints, see
https://gmplib.org/repo/gmp/rev/58879634af3ci



Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>