[LVu] base subseg head view on prev subseg's tail

Location views at borders between subsegments/subsections in the same
segment/section are computed as if each new subsegment/subsection
started with a forced view reset to zero, but the line number program
does not introduce resets that are not explicitly requested, so if a
subsegment ends at the same address another starts, the line number
program will have a continuity of views at the border address, whereas
the initial view number label in the latter subsegment will be
miscomputed as zero.

This patch delays the assignment of view expressions at subsegment
heads to the time of chaining the frags of subsegments into a single
segment, so that they are set based on the view at the end of the
previous subsegment in the same segment.

The line number program created for the test program had an
unnecessary DW_LNS_advance_pc at the end.  This patch also arranges
for us not to emit it.


for  gas/ChangeLog

	* dwarf2dbg.c (set_or_check_view): Skip heads when assigning
	views of prior locs.
	(dwarf2_gen_line_info_1): Skip heads.
	(size_inc_line_addr, emit_inc_line_addr): Drop
	DW_LNS_advance_pc for zero addr delta.
	(dwarf2_finish): Assign views for heads of segments.
	* testsuite/gas/elf/dwarf2-19.d: New.
	* testsuite/gas/elf/dwarf2-19.s: New.
	* testsuite/gas/elf/elf.exp: Test it.

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