gmp: fix target build for ARM thumb

Pull upstream hg changeset c56ea318eb64 and 676e2d0f0e4d to fix ARM
build breakage when compiling in thumb mode. Fixes:
http://autobuild.buildroot.net/results/f44/f447ce933ad8dd975b17aeda03cc46b0cace0b03/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>