gdb/testsuite: Allow DWARF assembler to create multiple line tables

gdb/testsuite: Allow DWARF assembler to create multiple line tables

Fixes a bug in the DWARF assembler that prevents multiple line tables
from being created in a test.  We currently don't initialise a couple
of flags, as a result we will only ever generate one end of file list,
and one end of header, in the first line table.  Any additional line
tables will be missing these parts, and will therefore be corrupt.

This fix will be required for a later commit.  There should be no
change in the testsuite after this commit.

gdb/testsuite/ChangeLog:

	* lib/dwarf.exp (Dwarf::lines): Reset _line_saw_program and
	_line_saw_file.

Change-Id: Id7123f217a036f26ee32d608db3064dd43164596