[gdb/testsuite] Add pseudo line number program instruction: line

[gdb/testsuite] Add pseudo line number program instruction: line

There's an idiom in dwarf assembly test-cases:
...
set line1 [gdb_get_line_number "line 1"]
set line2 [gdb_get_line_number "line 2"]
set line3 [gdb_get_line_number "line 3"]
  ...
            {DW_LNS_advance_line [expr $line1 - 1]}
  ...
            {DW_LNS_advance_line [expr $line2 - $line1]}
  ...
            {DW_LNS_advance_line [expr $line3 - $line2]}
...

Add a pseudo line number program instruction "line", such that we can simply
write:
...
            {line $line1}
  ...
            {line $line2}
  ...
            {line $line3}
...

Build and reg-tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-07-16  Tom de Vries  <tdevries@suse.de>

	* lib/dwarf.exp (program): Initialize _line.
	(DW_LNE_end_sequence): Reinitialize _line.
	(DW_LNS_advance_line): Update _line.
	(line): New proc.
	* gdb.dwarf2/dw2-inline-many-frames.exp: Use line.
	* gdb.dwarf2/dw2-inline-small-func.exp: Same.
	* gdb.dwarf2/dw2-inline-stepping.exp: Same.
	* gdb.dwarf2/dw2-is-stmt-2.exp: Same.
	* gdb.dwarf2/dw2-is-stmt.exp: Same.
	* gdb.dwarf2/dw2-ranges-func.exp: Same.