Ensure proc-service symbols have default visibility (PR build/24805)
    
    Compiling GDB with '-fvisibility=hidden' removes the symbols that
    should be exported.
    
    This patch explicitly marks them as visible.
    
    gdb/ChangeLog:
    2020-01-16  Pedro Alves  <palves@redhat.com>
                Norbert Lange  <nolange79@gmail.com>
    
    	PR build/24805
    	* gdbsupport/gdb_proc_service.h (PS_EXPORT): New.
    	(ps_get_thread_area, ps_getpid, ps_lcontinue, ps_lgetfpregs)
    	(ps_lgetregs, ps_lsetfpregs, ps_lsetregs, ps_lstop, ps_pcontinue)
    	(ps_pdread, ps_pdwrite, ps_pglobal_lookup, ps_pstop, ps_ptread)
    	(ps_ptwrite, ps_lgetxregs, ps_lgetxregsize, ps_lsetxregs)
    	(ps_plog): Redeclare exported functions with default visibility.