[gdb/testsuite] Remove Dwarf::extern

[gdb/testsuite] Remove Dwarf::extern

The file lib/dwarf.exp contains:
...
    # Declare a global label.  This is typically used to refer to
    # labels defined in other files, for example a function defined in
    # a .c file.
    proc extern {args} {
	foreach name $args {
	    _op .global $name
	}
    }
...

The assembler directive to refer to labels defined in other files is
not .global, but .extern, and that one is ignored by gas.

Since we require gas for all dwarf assembly test-cases, remove the proc and
all it's uses.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-07-17  Tom de Vries  <tdevries@suse.de>

	* lib/dwarf.exp (Dwarf::extern): Remove.
	* gdb.compile/compile-ops.exp: Remove use of Dwarf::extern.
	* gdb.dlang/circular.exp: Same.
	* gdb.dwarf2/comp-unit-lang.exp: Same.
	* gdb.dwarf2/dw2-ifort-parameter.exp: Same.
	* gdb.dwarf2/dw2-symtab-includes.exp: Same.
	* gdb.dwarf2/dwz.exp: Same.
	* gdb.dwarf2/imported-unit-abstract-const-value.exp: Same.
	* gdb.dwarf2/imported-unit-runto-main.exp: Same.
	* gdb.dwarf2/imported-unit.exp: Same.
	* gdb.dwarf2/opaque-type-lookup.exp: Same.