core: Fix build in ISO C++11 mode

Fix compatibility with ISO C++11 compilers

Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>
Change-Id: I8f9aa775b5681d4d8c5202a1a1935acb4efa4171
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>