libc: rename the threads options

Use a more coherent naming for the options. This will help commonalise
the native case (e.g. NPTL on Linux, win32 on Windows), and add alternate
implementations (e.g. musl.)

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Bryan Hundven <bryanhundven@gmail.com>
libc: libces are responsible for naming their threads implementation

This will help add new implementations, such as the one in musl.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Bryan Hundven <bryanhundven@gmail.com>
libc/mingw: threads are the native ones

There is no need to differentiate the win32 threads case, since we
can cosider them to be the native implementation on Windows.

Besides, with the previous patch, nothing uses it anymore.

So, just remove it.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc Bryan Hundven <bryanhundven@gmail.com>
libc/musl: add musl-libc support

This patch adds initial support for musl-libc.

Musl-libc versions currently supported:
* 1.0.3 (Stable)
* 1.1.3 (Previous Mainline)
* 1.1.4 (Mainline)

Futher improvements are needed.
* gcc-4.9.x has issues (Might be fixed in musl-1.1.4).
* Multilib support is needed.
* Checks to make sure paths are correct.
* Move to 2-step gcc build. 3-step build is not necessary.

Signed-off-by: Bryan Hundven <bryanhundven@gmail.com>
[yann.morin.1998@free.fr: removed the gcc musl patch, to be added later;
 removed dead code do_get_arch()]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>