filesystem_config: Allow executables in /system/libexec

Files in $PREFIX/libexec, such as gcc's cc1, need to be executable.
Don't let make_ext4fs strip executable permissions from them.

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
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).

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
corkscrew: Fix build with gcc 4.8

gcc 4.8 has a new warning for unused parameters - we need to override
-Werror on that.

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>