inline asm: Add new constraint for symbol definitions

The following patch on top of the PR41045 toplevel extended asm patch
allows marking inline asms (both toplevel and function-local, admittedly
it is less useful for the latter, so if you want, I can add restrictions)
as defining symbols, either functions or variables.

As most remaining constraint letters are used at least on some targets,
I'm using : as the new constraint.  It is similar to "s" in that it
wants CONSTANT_P && !CONST_SCALAR_INT_P, but
1) it specially requires an address of a function or variable declaration,
   so for functions the expected use is
void foo (void);
...
":" (foo)
or
":" (&foo)
and for variables (unless they are arrays)
extern int var;
...
":" (&var)
2) it makes no sense to say that either something is defined or it is
   used in a register or something similar, so the patch diagnoses if
   one attempts to mix it with other constraints; ":,:,:" is allowed
   just because one could be using 3 alternatives in some other operand
3) unlike "s", the constraint doesn't check LEGITIMATE_PIC_OPERAND_P for
   -fpic, even in -fpic one should be able to use it the same way
4) the cgraph portion needs to be really added later
5) and last but not least, I'm afraid %c0 print modifier isn't very
   good for printing it; it works fine without -fpic/-fpie, but 'c'
   modifier is handled as
                if (CONSTANT_ADDRESS_P (operands[opnum]))
                  output_addr_const (asm_out_file, operands[opnum]);
                else
                  output_operand (operands[opnum], 'c');
   and because at least on some arches like x86 CONSTANT_ADDRESS_P
   is redefined to do backend specific PIC mess, it will just
   output_operand and likely just be rejected (on x86 with an error
   that the argument is not a comparison)
   Guess for x86 one can use %p0 instead.
   But I'm afraid we are mostly out of generic modifiers,
   and targetm.asm_out.print_operand_punct_valid_p seems to use most
   of the punctuation characters as well.
   I think ` is unused, but wonder if we want to use up the last
   remaining letter that way, perhaps make %`<letter>0?
   Or extend the existing generic modifiers, keep %c0 behave as it
   does right now and make %cc0 be a 2 letter modifier which is
   PIC friendly and prints using output_addr_const anything that can
   be printed that way?  A follow-up patch implements the %cc0 version.

2024-12-18  Jakub Jelinek  <jakub@redhat.com>

gcc/
	* genpreds.cc (mangle): Add ':' mangling.
	(add_constraint): Allow : constraint.
	* common.md (:): New define_constraint.
	* stmt.cc (parse_output_constraint): Diagnose "=:".
	(parse_input_constraint): Handle ":" and diagnose invalid
	uses.
	* doc/md.texi (Simple Constraints): Document ":" constraint.
gcc/c/
	* c-typeck.cc (build_asm_expr): Diagnose invalid ":" constraint
	uses.
gcc/cp/
	* semantics.cc (finish_asm_stmt): Diagnose invalid ":" constraint
	uses.
gcc/testsuite/
	* c-c++-common/toplevel-asm-4.c: New test.
	* c-c++-common/toplevel-asm-5.c: New test.

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