llvm: stdint: Don't define integer constant macros in llvm.h

Zephyr minimal libc implements integer constant macros using internal
headers provided by GCC, for example INT64_C(x) is just a __INT64_C(x)
which is implemented by the GCC.

Clang compiler doesn't provide these definitions, so we define them
(__INT64_C() and INT64_C()) in zephyr/toolchain/llvm.h, but it looks
like INT64_C() definition doesn't come from llvm.h, but from stdlib.h
despite of checking if __GNUC__ is defined.

This is because Clang sometimes pretends to be GNU compiler:
$ x86_64-pc-linux-gnu-clang -dM -E - < /dev/null | grep "GNU"
 #define __GNUC_MINOR__ 2
 #define __GNUC_PATCHLEVEL__ 1
 #define __GNUC_STDC_INLINE__ 1
 #define __GNUC__ 4

Let's keep the integer constant macros in stdlib.h and their
actual implementation in llvm.h. Also define these macros if __clang__
is defined, just in case.

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