testsuite: Restore gdb_is_target_remote_prompt
    
    In patch
    
      Add test for load command
      3275ef477498e0500d7ea440f1bc51787acf4610
    
    I removed gdb_is_target_remote_prompt, but did not realize it was used
    in mi_is_target_remote.  This makes the gdb.mi/mi-nonstop.exp crash, for
    example:
    
      ERROR: (DejaGnu) proc "gdb_is_target_remote_prompt {[(]gdb[)]
      }" does not exist.
      The error code is TCL LOOKUP COMMAND gdb_is_target_remote_prompt
      The info on the error is:
      invalid command name "gdb_is_target_remote_prompt"
          while executing
      "::tcl_unknown gdb_is_target_remote_prompt {[(]gdb[)]
      }"
          ("uplevel" body line 1)
          invoked from within
      "uplevel 1 ::tcl_unknown $args"
    
    This patch restores it.
    
    gdb/testsuite/ChangeLog:
    
    	* lib/gdb.exp (gdb_is_target_1): Add prompt_regexp parameter and
    	use it.
    	(gdb_is_target_remote_prompt): New proc.
    	(gdb_is_target_remote): Use gdb_is_target_remote_prompt.
    	(gdb_is_target_native): Pass prompt parameter to
    	gdb_is_target_1.