Add some error checking to DWARF assembler

Add some error checking to DWARF assembler

I had written a DWARF location expression like

    DW_OP_const1u
    DW_OP_stack_value

... and was surprised to see that the DW_OP_stack_value didn't appear
in the "readelf" output.

The problem here is that DW_OP_const1u requires an operand, but
neither the DWARF assembler nor gas diagnosed this problem.

This patch adds some checking to Dwarf::_location to try to avoid this
in the future.  The checking is done via a helper proc that also
dissects the argument list and sets an array in the caller's frame.

gdb/testsuite/ChangeLog
2021-03-31  Tom Tromey  <tromey@adacore.com>

	* lib/dwarf.exp (Dwarf::_get_args): New proc.
	(Dwarf::_location): Use it.