Restructuring druntime

[Johannes Pfau]

As recently discussed in a pull request it would be great if we could
make it easier to port druntime to different architectures. Special
casing every C library, every architecture and every OS using version
blocks could lead to difficult to maintain code, so we need a better
solution.

As far as I can see, the biggest differences are caused by different C
libraries, not by different architectures (except 32bit vs 64 bit
differences). For example glibc headers seem to be very similar for arm
and x86, but bionic vs. glibc brings more differences. Bionic initially
didn't support anything wchar_t related, so core.stdc.wchar_ and
core.stdc.wctype couldn't work with android at all. And C libraries
vary even more in the subset of posix functionality they support.

We need a directory scheme to support
* Different C libraries
  * on different OS (glibc/bsd glibc/linux)
  * on the same OS (glibc on linux, bionic on linux, uclibc on
    linux, ...)
* Different architectures (ARM, X86, MIPS, PPC, SH4, ...)

I propose the following directory layout, but of course
better solutions are appreciated:

druntime
|-core
|---stdc
|---sync
|---sys
|-----posix
|-gc
|-gcstub
|-rt
|-gcc
|---arch
|-----glibc
|-------core
|---------stdc
|---------sync
|---------sys
|-----------posix
|-----bionic
|-------stdc
|-------sync
|-------core
|---------sys
|-----------posix
|-----newlib
|-------stdc
|-------sync
|-------core
|---------sys
|-----------posix

Code in the gc, gcstub and rt directories shouldn't depend on special C
libraries, small differences can be solved using version() blocks.

Every C library get's it's  own bindings in the gcc.arch package. I'm
not sure if those bindings should only provide functionality included
in core, or whether these bindings should include additional, C library
specific bindings.

The files in core.* would reference the corresponding files in
gcc.arch.core.*

The downside of that approach is that we'll have some code duplication,
especially for similar C libraries. And merging changes is therefore
not that easy as well. However, as the C and Posix headers don't change
often, that's probably neglectable.

Here are some examples for that directory scheme:

core/stdc/config.d (Should be shared with dmd druntime?)
-----------------------------------------------
enum LibC
{
    GlibC,
    uClibc,
    dietlibc,
    newlib,
    dmc, //digitalmars, windows
    msvcrt,
    ...,
    unknown
}

//Need some way to define this correctly
enum libC = LibC.GlibC;
enum string libCVersion = ""; //Free form version number
-----------------------------------------------

core/stdc/math.d
-----------------------------------------------
import core.stdc.config;

static if(libC == LibC.GlibC)
{
    public import gcc.arch.glibc.core.stdc.math;
}
else
{
    static assert(false, "C library '" ~ libC ~ "' not supported in " ~
    __FILE__);
}

//Maybe put generic parts here?
-----------------------------------------------

core/sys/posix/dirent.d
-----------------------------------------------
import core.stdc.config;

static if(libC == LibC.GlibC)
{
    /*
     * I admit this looks horrible. We can probably remove the core
     * part, maybe even gcc, so arch.glibc.sys.posix.dirent?
     * or glibc[.core].sys.posix.dirent?
     */
    public import gcc.arch.glibc.core.sys.posix.dirent;
}
else static if(libC == LibC.GlibC)
{
    public import gcc.arch.glibc.core.sys.posix.dirent;
}
else
{
    static assert(false, "C library '" ~ libC ~ "' not supported in " ~
    __FILE__);
}
-----------------------------------------------

Maybe we could use pragma(msg) warnings instead of static asserts, so
it'd be possible to import the modules in any case and use
traits(__compiles) to check for specific symbols/functions.