fortran: module-contained PRIVATE procedures must have global ELF linkage [PR125430]

Assisted-by: Claude Sonnet 4.6

gcc/fortran/ChangeLog:

	PR fortran/125430
	* trans-decl.cc (build_function_decl): Set TREE_PUBLIC for all
	module-contained procedures so submodules compiled as separate
	translation units can reach them via host association.  Also set
	DECL_VISIBILITY to VISIBILITY_HIDDEN for PRIVATE procedures,
	matching the existing treatment of module variables.

gcc/testsuite/ChangeLog:

	PR fortran/125430
	* gfortran.dg/module_private_2.f90: Remove scan-tree-dump-times
	assertion for 'priv'; PRIVATE module procedures now have global
	linkage with hidden visibility and are no longer optimized away.
	* gfortran.dg/public_private_module_2.f90: Add xfail markers to
	scan-assembler-not for 'two' and 'six'; update comment to mention
	procedures alongside variables.
	* gfortran.dg/public_private_module_7.f90: Add xfail marker to
	scan-assembler-not for '__m_common_attrs_MOD_other'.
	* gfortran.dg/public_private_module_8.f90: Add xfail marker to
	scan-assembler-not for '__m_MOD_myotherlen'.
	* gfortran.dg/submodule_private_host.f90: New test.
	* gfortran.dg/submodule_private_host_aux.f90: New auxiliary file.
	* gfortran.dg/warn_unused_function_2.f90: Remove 'defined but not
	used' expectation for s1; PRIVATE module procedures now have
	global linkage and no longer trigger the unused-function warning.

(cherry picked from commit 4d545cf40c23df3686bb9c0abdf5868a69ffe8fd)