ia64: Fix up -Wunused-parameter warning

Since r14-6945-gc659dd8bfb55e02a1b97407c1c28f7a0e8f7f09b
there is a warning
../../gcc/config/ia64/ia64.cc: In function ‘void ia64_start_function(FILE*, const char*, tree)’:
../../gcc/config/ia64/ia64.cc:3889:59: warning: unused parameter ‘decl’ [-Wunused-parameter]
 3889 | ia64_start_function (FILE *file, const char *fnname, tree decl)
      |                                                      ~~~~~^~~~
which presumably for bootstraps breaks the bootstrap.
While the decl parameter is passed to the ASM_OUTPUT_FUNCTION_LABEL macro,
that macro actually doesn't use that argument, so the removal of
ATTRIBUTE_UNUSED was incorrect.
This patch reverts the first ia64.cc hunk from r14-6945.

2024-01-23  Jeff Law  <jlaw@ventanamicro.com>
	    Jakub Jelinek  <jakub@redhat.com>

	* config/ia64/ia64.cc (ia64_start_function): Add ATTRIBUTE_UNUSED to
	decl.