include/toolchain/gcc.h: Fix static assert detection

The logic for using _Static_assert() was a little broken.  We were
using it when on GCC 4.6+ AND when __STDC_VERSION__ said we were on
C99 or better.  But it's not a C99 feature, it's a C11 feature.  And
if GCC provides it as an extension, that's unrelated to a particular
language version.  This should have been "GCC 4.6+ OR C11+".

This actually broke on the ESP-32 IDF toolchain, where (when using
-std=c99) the compiler was actually defining a C99 macro instead of
the C11 one, and choosing to use the wrong (and independently broken)
handling incorrectly.  Fixes #8093.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>