New configuration option

[J. Johnston]

I have added a new configuration option: --disable-newlib-supplied-syscalls.
This is supposed to disable newlib from including syscall code in libc.  At the
moment, only the libc/sys/arm directory is using the option to decide
whether to build syscalls.o or not.  This option will be used to handle
older newlib ports where the syscalls were added in libc/sys instead of
libgloss where they now should reside.  I will modify other such older
newlib ports in the near future.

-- Jeff J.