SYCL_EXTERNAL definition is inconsistent.

### Describe the bug

Here is an example of the code to demonstrate the inconsistency:
```console
$ cat example.cpp
SYCL_EXTERNAL int mul(int x) {
return x * 42;
}
$ clang++ -fsycl-device-only example.cpp
$ clang++ -fsycl-host-only example.cpp
example.cpp:1:1: error: unknown type name 'SYCL_EXTERNAL'
1 | SYCL_EXTERNAL int mul(int x) {
| ^
1 error generated.
$ clang++ -fsycl example.cpp
example.cpp:1:1: error: unknown type name 'SYCL_EXTERNAL'
1 | SYCL_EXTERNAL int mul(int x) {
| ^
1 error generated.
```

Device compiler defines `SYCL_EXTERNAL` macro, whereas host compiler does not.

To define the macro on the host, `defines_elementary.hpp` header must be included. https://github.com/intel/llvm/blob/sycl/sycl/include/sycl/detail/defines_elementary.hpp#L39

I found this issue while trying to create a simple library of C++ functions available from the SYCL kernels. My library code does not use any SYCL data structures or any symbols. It uses only `SYCL_EXTERNAL` macro.
I think the implementation should define this macro consistently either using the compiler or using the library, but not both.

@gmlueck, @tahonermann, @Fznamznon, what do you think?

My personal preference is to use the compiler to define `SYCL_EXTERNAL`.

### To reproduce

_No response_

### Environment

_No response_

### Additional context

_No response_