gdb: Handle multiple base address in debug_ranges data.

gdb: Handle multiple base address in debug_ranges data.

It is possible to use multiple base addresses within a single address
range series, within the .debug_ranges section.  The following is a
simplified example for 32-bit addresses:

  .section ".debug_ranges"
  .4byte	0xffffffff
  .4byte	BASE_1
  .4byte	START_OFFSET_1
  .4byte	END_OFFSET_1
  .4byte	START_OFFSET_2
  .4byte	END_OFFSET_2
  .4byte	0xffffffff
  .4byte	BASE_2
  .4byte	START_OFFSET_3
  .4byte	END_OFFSET_3
  .4byte	0
  .4byte	0

In this example START/END 1 and 2 are relative to BASE_1, while
START/END 3 are relative to BASE_2.

Currently gdb does not correctly parse this DWARF, resulting in
corrupted address range information.  This commit fixes this issue, and
adds a new test to cover this case.

In order to support testing of this feature extensions were made to the
testsuite dwarf assembler, additional functionality was added to the
.debug_line generation function, and a new function for generating the
.debug_ranges section was added.

gdb/ChangeLog:

	* dwarf2read.c (dwarf2_ranges_read): Unify and fix base address
	reading code.

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/dw2-ranges-base.c: New file.
	* gdb.dwarf2/dw2-ranges-base.exp: New file.
	* lib/dwarf.exp (namespace eval Dwarf): Add new variables to
	support additional line table, and debug ranges generation.
	(Dwarf::ranges): New function, generate .debug_ranges.
	(Dwarf::lines): Support generating simple line table programs.
	(Dwarf::assemble): Initialise new namespace variables.