core.stdc and betterC

[dd886k]

Hello!

This is my first time posting, so do feel free to correct me and 
this post.

I started writing in D around Q2 2017 and recently been 
re-writing some of my projects as betterC (entirely!).

I noticed something weird. It started with DMD 2.074.0 and it's 
still an issue for me in DMD 2.079.1 (and any other compiler 
really).

Consider this piece:

```
import core.stdc.stdio;

extern(C) void main() {
	printf("a");
}
```

It compiles and runs absolutely wonderful (OMF, MSCOFF, ELF, 
etc.). No issues.

Now consider this piece:

```
import core.stdc.stdio;

extern(C) void main() {
	putchar('a');
}
```

Oops, `Error 42: Symbol Undefined 
__D4core4stdc5stdio7putcharFNbNiNeiZi`! To my surprise, `putchar` 
is extern'd as D in druntime/src/core/stdc/stdio.d, and I find 
that _really silly_.

Which means `putchar`, during linking, is affected for linking 
every single C runtime out there, and the only reason I can think 
of is to easily integrate it with D in general, which in my 
opinion, is not necessarily needed since the D people (not meant 
as an insult) promotes betterC as a stand-alone option.

On Windows, stdin, stdout, and stderr are affected when using 
-m32mscoff and -m64 (and obviously, LDC) because under 
CRuntime_Microsoft, std* are defined as `shared`.

I'm aware that an easy solution would be defining a version 
(D_betterC) section.

Will I do a Pull Request? Unfortunately no, I fear I'll abandon 
mid-way through. It's easier to advise the forums and let an 
actual professional integrate the fix.

If you have other suggestions, I'm all ears.