Specify source file explicitly when setting a breakpoint

When I run no-thread-db.exp, the breakpoint is set on line 26.
However, the breakpoint is set to line 26 of dl-start.S rather than
no-thread-db.c, which is not intended.

(gdb) monitor set libthread-db-search-path /foo/bar^M
libthread-db-search-path set to `/foo/bar'^M
(gdb) PASS: gdb.server/no-thread-db.exp: libthread-db is now unresolvable
break 26^M
Breakpoint 1 at 0x48018078: file ../sysdeps/powerpc/powerpc32/dl-start.S, line 26.^M
(gdb) continue^M
Continuing.

This patch is to change the breakpoint setting with source file
specified, then it is correct now.

gdb/testsuite:

2014-05-26  Yao Qi  <yao@codesourcery.com>

	* gdb.server/no-thread-db.exp: Specify source file name
	explicitly when setting a breakpoint.