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>