Increase timeout in watch-bitfields.exp for software watchpoint

Increase timeout in watch-bitfields.exp for software watchpoint

I see the following two timeout fails on pandaboard (arm-linux target),

 FAIL: gdb.base/watch-bitfields.exp: -location watch against bitfields: continue until exit (timeout)
 FAIL: gdb.base/watch-bitfields.exp: regular watch against bitfields: continue until exit (timeout)

In this test, more than one watchpoint is used, so the following
watchpoint requests fall back to software watchpoint, so that GDB
will single step all the way and it is very slow.

This patch is to copy the fix from

  [PATCH] GDB/testsuite: Correct gdb.base/watchpoint-solib.exp timeout tweak
  https://sourceware.org/ml/gdb-patches/2014-07/msg00716.html

I find the left-over of this patch review is to factor out code into
a procedure, so I do that in this patch.

Re-run tests watch-bitfields.exp, watchpoint-solib.exp, sigall-reverse.exp,
and until-precsave.exp on pandaboard, no regression.

gdb/testsuite:

2015-04-15  Pedro Alves  <palves@redhat.com>
	    Yao Qi  <yao.qi@linaro.org>

	* gdb.base/watch-bitfields.exp (test_watch_location): Increase
	timeout by factor of 4.
	(test_regular_watch): Likewise.
	* gdb.base/watchpoint-solib.exp: Use with_timeout_factor.
	* gdb.reverse/sigall-reverse.exp: Likewise.
	* gdb.reverse/until-precsave.exp: Likewise.
	* lib/gdb.exp (with_timeout_factor): New proc.
	(gdb_expect): Move some code to ...
	(get_largest_timeout): ... here.  New procedure.