Eliminate literal line numbers in foll-exec.exp

Eliminate literal line numbers in foll-exec.exp

Remove literal line numbers from the regexps in foll-exec.exp.  Add
appropriate eye-catchers to foll-exec.c and execd-proc.c and refer to
those instead.

gdb/testsuite/ChangeLog:

	* gdb.base/execd-prog.c: Add eye-catchers.
	* gdb.base/foll-exec.c: Likewise.
	* gdb.base/foll-exec.exp: Refer to eye-catchers instead of literal
	line numbers.