gdb.base/async.exp: Don't hardcode line numbers.

gdb.base/async.exp: Don't hardcode line numbers.

gdb/testsuite/
2014-03-19  Pedro Alves  <palves@redhat.com>

	* gdb.base/async.c (main): Add "jump here" and "until here" line
	marker comments.
	* gdb.base/async.exp (jump_here): New global.
	(jump& test): Use it.
	(until_here): New global.
	(until& test): Use it.