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

The redundant -md option was removed in e4ae357f, 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.