libc/newlib: add a config option allowing to disable builtin syscalls

Allows to choose if one wants to keep or not the syscalls that are provided with
newlib. It passes the --disable-newlib-supplied-syscalls or
--enable-newlib-supplied-syscalls to the configure script. If one chooses to
disable the builtin syscalls, he/she will have to write his/her own. This can
be usefull to port newlib to a new platform/board.

Signed-off-by: Kévin PETIT <kpet@free.fr>

 # Bare metal config options

 ## select BARE_METAL

 ##

 ## help 'Bare metal' refers to those programs that run without any kernel.

 ## help

 ## help You probably want to say 'y' here if you plan to use your compiler

 ## help to build bootloaders. It is not yet suitable to build Linux kernels,

 ## help though, because the APCI stuff relies on the target C library headers

 ## help being available?!?!...