Clean up gdb.trace/entry-values.exp

Clean up gdb.trace/entry-values.exp

This patch is to clean up gdb.trace/entry-values.exp as a preparation
of the next patch.  It updates the comments to reflect the code.

One DIE generated in dwarf assembler is

 		GNU_call_site {
		    {low_pc "$bar_start + $bar_call_foo" addr}
 		    {abstract_origin :$foo_label}

the DW_AT_low_pc attribute is the return address after the call, so I
rename variable bar_call_foo to returned_from_foo.

gdb/testsuite:

2014-12-29  Yao Qi  <yao@codesourcery.com>

	* gdb.trace/entry-values.exp: Update comments.  Rename variable
	bar_call_foo to returned_from_foo.