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>

 # s390 specific config options

 ## depends on EXPERIMENTAL

 ##

 ## select ARCH_SUPPORTS_32

 ## select ARCH_SUPPORTS_64

 ## select ARCH_DEFAULT_32

 ## select ARCH_USE_MMU

 ## select ARCH_SUPPORT_FPU

 ##

 ## help Defined by IBM in:

 ## help     32-bit ESA/390       : http://publibfp.boulder.ibm.com/cgi-bin/bookmgr/BOOKS/dz9ar008/CCONTENTS

 ## help     64-bit z/Architecure : http://publibfp.boulder.ibm.com/cgi-bin/bookmgr/BOOKS/dz9zr002/CCONTENTS