Darwin: Fix constant CFString code-gen [PR105522].

Darwin: Fix constant CFString code-gen [PR105522].

Although this only fires for one of the Darwin sub-ports, it is latent
elsewhere, it is also a regression c.f. the Darwin system compiler.

In the code we imported from an earlier branch, CFString objects (which
are constant aggregates) are constructed as CONST_DECLs.  Although our
current documentation suggests that these are reserved for enumeration
values, in fact they are used elsewhere in the compiler for constants.
This includes Objective-C where they are used to form NSString constants.

In the particular case, we take the address of the constant and that
triggers varasm.cc:decode_addr_constant, which does not currently support
CONST_DECL.

If there is a general intent to allow/encourage wider use of CONST_DECL,
then we should fix decode_addr_constant to look through these and evaluate
the initializer (a two-line patch, but I'm not suggesting it for stage-4).

We also need to update the GCC internals documentation to allow for the
additional uses.

This patch is Darwin-local and fixes the problem by making the CFString
constants into regular variable but TREE_CONSTANT+TREE_READONLY. I plan
to back-port this to the open branches once it has baked a while on trunk.

Since, for Darwin, the Objective-C default is to construct constant
NSString objects as CFStrings; this will also cover the majority of cases
there (this patch does not make any changes to Objective-C NSStrings).

	PR target/105522

gcc/ChangeLog:

	* config/darwin.cc (machopic_select_section): Handle C and C++
	CFStrings.
	(darwin_rename_builtins): Move this out of the CFString code.
	(darwin_libc_has_function): Likewise.
	(darwin_build_constant_cfstring): Create an anonymous var to
	hold each CFString.
	* config/darwin.h (ASM_OUTPUT_LABELREF): Handle constant
	CFstrings.

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