Eliminate literal line numbers in mi-until.exp

Eliminate literal line numbers in mi-until.exp

Remove literal line numbers from the regexps in mi-until.exp.  Add
appropriate eye-catchers to until.c and refer to those instead.

This change fixes the test case after having disturbed the line
numbering with the previous fix for compiler warnings with -std=gnu11.

gdb/testsuite/ChangeLog:

	* gdb.mi/until.c: Add eye-catchers.
	* gdb.mi/mi-until.exp: Refer to eye-catchers instead of literal
	line numbers.