Darwin, Driver: Avoid a link line for empty commands.

We were pushing a spec value for weak_reference_mismatches unconditionally
which is not needed (the value was the default) and the side-effect of
this was that we appeared to need to drive a link command; leading to
unexpected diagnostics for cases where gcc was invoked with an empty
command line.

Also we were pushing flags for sysroot, os minimum version and controls
even if the command line was empty.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

gcc/ChangeLog:

	* config/darwin-driver.c (darwin_driver_init): Exit from the
	option handling early if the command line is definitely enpty.
	* config/darwin.h (SUBTARGET_DRIVER_SELF_SPECS): Remove
	setting for the default content of weak_reference_mismatches.