MSP430: gas: Ignore -md option required for GCC backward compatibility

MSP430: gas: Ignore -md option required for GCC backward compatibility

The redundant -md option was removed in e4ae357fe8, but it is required
for backwards compatibility with GCC 10, which passes it to the
assembler implicitly in certain situations.

It is now silently ignored.

gas/ChangeLog:

	* config/tc-msp430.c (OPTION_MOVE_DATA): Define.
	(md_parse_option): Ignore OPTION_MOVE_DATA.
	(md_longopts): Handle -md option.
	* testsuite/gas/msp430/msp430.exp: Run new test.
	* testsuite/gas/msp430/empty.s: New test.
	* testsuite/gas/msp430/ignore-md.d: New test.