Fix GCC optimization guards for Clang

[ricardoasouz]

Fixes #205

Clang defines __GNUC__ for GCC compatibility, but it does not support the GCC-specific optimize attribute and #pragma GCC optimize used in these code paths.

This change excludes Clang from the GCC-only optimization hints and also adds the missing IAR guard for arm_depthwise_conv_s8.c.

Validation:

• Reproduced the Clang warnings for both the optimize attribute and #pragma GCC optimize.
• Verified the guards remain enabled for GCC.
• Verified the guards are disabled for Clang.
• Completed the official CMSIS-NN Cortex-M33 GCC build successfully.