Use DW_FORM_exprloc in testsuite Dwarf Assembler for DWARF version 4+.

Use DW_FORM_exprloc in testsuite Dwarf Assembler for DWARF version 4+.

Since DWARF version 4 expressions are represented by DW_FORM_exprloc
instead of a block form. Support this in the testsuite Dwarf Assembler
by setting the SPECIAL_expr form once we know the CU version.

This doesn't change any testsuite results, it just makes the produced
DWARF valid. gdb also accepts expressions in block form for DWARF
version 4 and above, but this is technically incorrect.

gdb/testsuite/ChangeLog:

	* lib/dwarf.exp (Dwarf::_read_constants): Don't set
	_constants(SPECIAL_expr) here, but set it...
	(Dwarf::cu): ...here based on _cu_version.