Move step_until procedure

Procedure step_until from test gdb.reverse/step-indirect-call-thunk.exp
is moved to lib/gdb.exp and renamed repeat_cmd_until.  The existing procedure
gdb_step_until in lib/gdb.exp is simpler variant of the new repeat_cmd_until
procedure.  The existing procedure gdb_step_until is changed to just call
the new repeat_cmd_until procedure with the command set to "step" and an
optional CURRENT string.  The default CURRENT string is set to "\}" to work
with the existing uses of procedure gdb_step_until.