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>

 # Overall toolchain configuration: paths, jobs, etc...

 # Allow unconditional usage of tristates

 config MODULES

     bool

     default y

 menu "Paths and misc options"

 source "config/global/ct-behave.in"

 source "config/global/paths.in"

 source "config/global/download.in"

 source "config/global/extract.in"

 source "config/global/build-behave.in"

 source "config/global/logging.in"

 endmenu