Treat optimize attribute as gcc specific

Clang doesn't provide support for __attribute__((optimize ...)). There
are alternatives to provide the same functionality but due to how rarely
the kernel is compiled without optimisations, this can be added on
demand.

include/util.h

@@ -48,7 +48,11 @@
 #define UNUSED       __attribute__((unused))
 #define USED         __attribute__((used))
 #define FASTCALL     __attribute__((fastcall))
+#ifdef __clang__
+#define FORCE_O2     /* nothing */
+#else
 #define FORCE_O2     __attribute__((optimize("O2")))
+#endif
 /** MODIFIES: */
 void __builtin_unreachable(void);
 #define UNREACHABLE()  __builtin_unreachable()

src/arch/arm/32/idle.c

@@ -19,6 +19,8 @@
  *  - All nested function calls are inlined
  * Note that GCC does not correctly implement optimisation annotations on nested
  * functions, so FORCE_INLINE is required on the wfi declaration in this case.
+ * Note that Clang doesn't obey FORCE_O2 and relies on the kernel being compiled
+ * with optimisations enabled.
  */
 void FORCE_O2 idle_thread(void)
 {