Fix gdb.trace/entry-values.exp for thumb mode

Fix gdb.trace/entry-values.exp for thumb mode

We see some fails in gdb.trace/entry-values.exp in thumb mode
(-mthumb -march={armv4t,armv7-a}).

In thumb mode, the lsb of references to 'foo' and 'bar' in the assembly
(produced by dwarf assember) is set, so the generated debug
information is incorrect.

This patch copies the approach used by

  [PATCH 4/4] Fix dw2-ifort-parameter.exp on PPC64
  https://sourceware.org/ml/gdb-patches/2014-03/msg00202.html

to introduce new labels 'foo_start' and 'bar_start' which are about
the correct function address (without lsb set).  This patch fixes
these fails we've seen.

gdb/testsuite:

2014-07-08  Yao Qi  <yao@codesourcery.com>

	* gdb.trace/entry-values.c: Define labels 'foo_start' and
	'bar_start' at the beginning of functions 'foo' and 'bar'
	respectively.
	* gdb.trace/entry-values.exp: Use 'foo_start' and 'bar_start'
	instead of 'foo' and 'bar'.