MSP420 assembler: Add -m{u,U} options to enable/disable NOP warnings for unknown...

MSP420 assembler:  Add -m{u,U} options to enable/disable NOP warnings for unknown interrupt state changes

gas	* config/tc-msp430.c (options): New OPTION_UNKNOWN_INTR_NOPS,
	OPTION_NO_UNKNOWN_INTR_NOPS and do_unknown_interrupt_nops.
	(md_parse_option): Handle OPTION_UNKNOWN_INTR_NOPS and
	OPTION_NO_UNKNOWN_INTR_NOPS by setting do_unknown_interrupt_nops
	accordingly.
	(md_show_usage): Likewise.
	(md_shortopts): Add "mu" for OPTION_UNKNOWN_INTR_NOPS and
	"mU" for OPTION_NO_UNKNOWN_INTR_NOPS.
	(md_longopts): Likewise.
	(warn_eint_nop): Update comment.
	(warn_unsure_interrupt): Don't warn if prev_insn_is_nop or
	prev_insn_is_dint or we are assembling for 430 ISA.
	(msp430_operands): Only call warn_unsure_interrupt if
	do_unknown_interrupt_nops == TRUE.
	* testsuite/gas/msp430/nop-unknown-intr.s: New test source file.
	* testsuite/gas/msp430/nop-unknown-intr-430.d: New test.
	* testsuite/gas/msp430/nop-unknown-intr-430x.d: New test.
	* testsuite/gas/msp430/nop-unknown-intr-430x-ignore.d: New test.
	* testsuite/gas/msp430/nop-unknown-intr-430.l: Warning output for new
	test.
	* testsuite/gas/msp430/nop-unknown-intr-430x.l: Likewise.
	* testsuite/gas/msp430/msp430.exp: Add new tests to driver.