fortran: [PR126872] BIND(C) entities must keep default ELF visibility

Assisted-by: Claude Opus 5

A binding label gives an entity external linkage, so PRIVATE hides only
the Fortran name and must not lower the symbol's ELF visibility.  Three
sites gave hidden visibility to PRIVATE module entities without exempting
binding labels, so a BIND(C) procedure or variable in a module with a
PRIVATE default was not exported from a shared library.

The procedure case is a regression from r17-913 (PR fortran/125430),
backported to the 16 branch as r16-9113 and so present in 16.2.0 but not
16.1.0; the variable case dates from r6-2637, the introduction of
submodules (PR fortran/52846).  Module entities
without a binding label keep the hidden visibility that lets submodules
reach them via host association.

gcc/fortran/ChangeLog:

	PR fortran/126872
	* trans-decl.cc (gfc_finish_var_decl): Drop the unreachable
	hidden-visibility case in the BIND(C) block.  Do not give hidden
	visibility to a PRIVATE module variable with a binding label.
	(build_function_decl): Do not give hidden visibility to a PRIVATE
	module procedure with a binding label.

gcc/testsuite/ChangeLog:

	PR fortran/126872
	* gfortran.dg/bind_c_private_1.f90: New test.

(cherry picked from commit 75136eabafd2028a04d47720ea87548df47e2578)