Revert all usages of gcc's __extension__ keyword, and instead assert

Revert all usages of gcc's __extension__ keyword, and instead assert
that our headers are not built with __GNUC__ and __STRICT_ANSI__.

__extension__ had some weird (and buggy) behaviours when nested which
make it a risky proposition for general usage in our header
files. Better to disallow -ansi, -std=c99, and similar gcc options
when building against Xen headers.

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>