[gdb/testsuite] Fix .debug_abbrev terminators

[gdb/testsuite] Fix .debug_abbrev terminators

The abbreviations table for a single compilation unit has two types of
terminators:
- a ".byte 0" pair denoting the end of an attribute list
- a single ".byte 0" denoting the end of the table

However, at the end of the .debug_abbrev section in dw2-line-number-zero-dw.S,
we have four ".byte 0" entries:
...
        .uleb128        0x12            /* DW_AT_high_pc */
        .uleb128        0x01            /* DW_FORM_addr */
        .byte        0x0                /* Terminator */
        .byte        0x0                /* Terminator */
        .byte        0x0                /* Terminator */
        .byte        0x0                /* Terminator */
...

The first two are the attribute list terminator, the third is the end-of-table
terminator, and the last is superfluous/incorrect.

Fix this by emitting instead:
...
        .uleb128        0x12            /* DW_AT_high_pc */
        .uleb128        0x01            /* DW_FORM_addr */
        .byte        0x0                /* DW_AT - Terminator */
        .byte        0x0                /* DW_FORM - Terminator */
        .byte        0x0                /* Abbrev end - Terminator */
...
where the last comment resembles the comment for other abbreviation codes:
...
        .section .debug_abbrev
.Labbrev1_begin:
        .uleb128        2               /* Abbrev start */
...

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-11-03  Tom de Vries  <tdevries@suse.de>

	* lib/dwarf.exp (Dwarf::_handle_DW_TAG): Improve attribute list
	terminator comments.
	(Dwarf::cu, Dwarf::tu): Remove superfluous abbreviation table
	terminator.