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.