gcc: fix conditional move constraint on ARC

The patch solves one of the ARC internal compiler issues.

Fixes:
  http://autobuild.buildroot.net/results/1c4493c6a18b7a93a0368420c8f79032f9330891/
  http://autobuild.buildroot.net/results/51071b790866b0c7083de3a94ad2a427009096af/
  http://autobuild.buildroot.net/results/09e037c628ca2a27d8eb832dd070f50ec7ebb72a/



Signed-off-by: Vlad Zakharov <vzakhar@synopsys.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>