Darwin, Objective-C: Support -fconstant-cfstrings [PR108743].

This support the -fconstant-cfstrings option as used by clang (and
expect by some build scripts) as an alias to the target-specific
-mconstant-cfstrings.

The documentation is also updated to reflect that the 'f' option is
only available on Darwin, and to add the 'm' option to the Darwin
section of the invocation text.

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

	PR target/108743

gcc/ChangeLog:

	* config/darwin.opt: Add fconstant-cfstrings alias to
	mconstant-cfstrings.
	* doc/invoke.texi: Amend invocation descriptions to reflect
	that the fconstant-cfstrings is a target-option alias and to
	add the missing mconstant-cfstrings option description to the
	Darwin section.