corkscrew-x86: Fix build with some glibc versions

Make sure __USE_GNU is actually set before ucontext.h is included
(even implicitly).
This should fix the build inside android-build (wasn't broken locally
in the first place).

Change-Id: Ic8fba210df502c3017e53a8ca4961c4a5815df67
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>