gmp: Add workaround for ARM as bug

Current ARM as treats whitespace between a "[" and its content as a
fatal error.
Adapt the code to build even with this version.

Change-Id: Ic8cc0f4cb55c072568dc4ffa807b9b82a8561504
Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>
1 file changed