Eliminate literal line numbers in mi-console.exp

Eliminate literal line numbers in mi-console.exp

Remove the literal line number from a regexp in mi-console.exp.  Add
an appropriate eye-catcher to mi-console.c and refer to that instead.

gdb/testsuite/ChangeLog:

	* gdb.mi/mi-console.c: Add eye-catcher.
	* gdb.mi/mi-console.exp (semihosted_string): Refer to eye-catcher
	instead of literal line number.