gdb/testsuite: Add is-stmt support to the DWARF compiler

gdb/testsuite: Add is-stmt support to the DWARF compiler

This commit adds the ability to set and toggle the DWARF line table
is-stmt flag.

A DWARF line table can now be created with the attribute
'default_is_stmt' like this:

  lines {version 2 default_is_stmt 0} label {
    ...
  }

If 'default_is_stmt' is not specified then the current default is 1,
which matches the existing behaviour.

Inside the DWARF line table program you can now make use of
{DW_LNS_negate_stmt} to toggle the is-stmt flag, for example this
meaningless program:

  lines {version 2 default_is_stmt 0} label {
    include_dir "some_directory"
    file_name "some_filename" 1

    program {
      {DW_LNS_negate_stmt}
      {DW_LNE_end_sequence}
    }
  }

This new functionality will be used in a later commit.

gdb/testsuite/ChangeLog:

	* lib/dwarf.exp (Dwarf::lines) Add support for modifying the
	is-stmt flag in the line table.

Change-Id: Ia3f61d523826382dd2333f65b9aae368ad29c4a5