cmake: Define WAFFLE_HAS_TLS_MODEL_INITIAL_EXEC if compiler supports it
cmake: Define WAFFLE_HAS_TLS_MODEL_INITIAL_EXEC if compiler supports it

That is, if the compiler supports `__attribute__(tls_model("initial-exec"))`.

Signed-off-by: Chad Versace <a557f7f75bf28dc19ac99107f5cc7cd6b3fbdb1a@linux.intel.com>

janesma/waffle,waffle-gl/waffle,dcbaker/waffle,maurossi/waffle,evelikov/waffle,fjhenigman/waffle,waffle-gl/waffle,fjhenigman/waffle,yinquan529/waffle-gl-waffle,yinquan529/waffle-gl-waffle,janesma/waffle,janesma/waffle,dcbaker/waffle,maurossi/waffle,evelikov/waffle,yinquan529/waffle-gl-waffle,maurossi/waffle,chadversary/waffle,fjhenigman/waffle,dcbaker/waffle,evelikov/waffle,waffle-gl/waffle,waffle-gl/waffle,chadversary/waffle,chadversary/waffle