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>

# Alpha specific configuration file

## select ARCH_SUPPORTS_32

## select ARCH_DEFAULT_32

## select ARCH_USE_MMU

## select ARCH_SUPPORT_CPU

## select ARCH_SUPPORT_TUNE

##

## help The Alpha architecture.