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>

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