toolchain/llvm: Provide integer macros only when necessary

Using '#ifndef' before defining a macro is a good way to provide default
version of the macro if not defined elsewhere. But it has some
disadvantages:
- It hides information about the users of these macros. It's hard to
  determine when this implementation is used.
- Correctness depends on file including order.

It looks like these macros are used only by minimal libc and only if
ENFORCE_ZEPHYR_STDINT is not selected. We expect other libc to provide
their own implementation.

Signed-off-by: Patryk Duda <patrykd@google.com>

include/zephyr/toolchain/llvm.h

@@ -30,46 +30,27 @@
 
 #include <zephyr/toolchain/gcc.h>
 
-#ifndef __INT8_C
+/*
+ * Provide these definitions only when minimal libc is used.
+ * Avoid collision with defines from include/zephyr/toolchain/zephyr_stdint.h
+ */
+#ifdef CONFIG_MINIMAL_LIBC
+#ifndef CONFIG_ENFORCE_ZEPHYR_STDINT
+
 #define __INT8_C(x)	x
-#endif
-
-#ifndef __UINT8_C
 #define __UINT8_C(x)	x ## U
-#endif
-
-#ifndef __INT16_C
 #define __INT16_C(x)	x
-#endif
-
-#ifndef __UINT16_C
 #define __UINT16_C(x)	x ## U
-#endif
-
-#ifndef __INT32_C
 #define __INT32_C(x)	x
-#endif
-
-#ifndef __UINT32_C
 #define __UINT32_C(x)	x ## U
-#endif
-
-#ifndef __INT64_C
 #define __INT64_C(x)	x
-#endif
-
-#ifndef __UINT64_C
 #define __UINT64_C(x)	x ## ULL
-#endif
 
-#ifndef __INTMAX_C
+#endif /* !CONFIG_ENFORCE_ZEPHYR_STDINT */
+
 #define __INTMAX_C(x)	x
-#endif
-
-
-#ifndef __UINTMAX_C
 #define __UINTMAX_C(x)	x ## ULL
-#endif
 
+#endif /* CONFIG_MINIMAL_LIBC */
 
 #endif /* ZEPHYR_INCLUDE_TOOLCHAIN_LLVM_H_ */