libc/newlib: build in the 'start files' pass

A few facts:
- building the C library requires a proper core compiler
- core compiler is issued from one of the core passes
- the C library is required to build libstdc++
- newlib is only built for baremetal
- in bare metal, the final compiler is issued from one of the core passes

So we need to build the C library between core pass 1 and core pass 2.
The only place is eithe libc_headers() or libc_start_files(). The most
pertinent seems to be libc_start_files().

So we build newlib from libc_start_files(), and leave libc() empty.

cc/gcc: store core build rules in an array

Using an array makes it easier to add new rules.
Besides, it is easy to expand from build rules to install rules

cc/gcc: implement default for core options

cc/gcc: fix headers copy for core static

complibs/mpfr: update to latest version

libc/uClibc: apply the threading model to the configuration

The threading model shall be be set in the .config file.
Also, offer the choice between 'old/stable' and 'new' linuxthreads.

config: add support for a second part of the generated choices

Some components have configuration options that can depend on
generic options, so they should go below those.

uClibc for example:
- has its own options (wchar...)
- uses the generic options (threads...)
- if linuxthreads chosen, offers two impls

So we need to be able to split the components options in 2,
one part that is above the generic options, and one part that
ends up below the generic options.

docs: update credtis, point to the repository log

The docs/CREDITS file dates back to the SVN repository.
Now that we use Mercurial, the repository stores appropriate
authorship for each commit. Say so in the CREDITS.

kernel/linux: update to latest versions

kconfig/lxdialog: more portable and use ncurses from macports on MacOS

check-lxdialog.sh now correctly tests for ncurses and is a bit more portable.
On Darwin it uses the ncurses from macports.