[gdb/symtab] Ignore DW_LNE_lo_user/DW_LNE_hi_user range

When reading an exec with a .debug_line section containing a vendor-specific
extended opcode, we get:
...
$ gdb -batch -iex "set complaints 10" dw2-vendor-extended-opcode
During symbol reading: mangled .debug_line section
...
and reading of the .debug_line section is abandoned.

The vendor-specific extended opcode should be ignored, as specified in the
DWARF standard (7.1 Vendor Extensibility).  [ FWIW, vendor-specific
standard opcodes are already ignored. ]

Fix this by ignoring all vendor-specific extended opcodes.

Build and tested on x86_64-linux.

gdb/ChangeLog:

2020-08-03  Tom de Vries  <tdevries@suse.de>

	PR symtab/26333
	* dwarf2/read.c (dwarf_decode_lines_1): Ignore
	DW_LNE_lo_user/DW_LNE_hi_user range.

gdb/testsuite/ChangeLog:

2020-08-03  Tom de Vries  <tdevries@suse.de>

	PR symtab/26333
	* lib/dwarf.exp (DW_LNE_user): New proc.
	* gdb.dwarf2/dw2-vendor-extended-opcode.c: New test.
	* gdb.dwarf2/dw2-vendor-extended-opcode.exp: New file.