MSP430: Fix detection of assembler support for .mspabi_attribute

MSP430: Fix detection of assembler support for .mspabi_attribute

The assembly code ".mspabi_attribute 4,1" uses the object attribute
mechanism to indicate that the 430 ISA is in use. However, the default
ISA is 430X, so GAS fails to assemble this since the ISA wasn't also set
to 430 on the command line.

gcc/ChangeLog:

	* config/msp430/msp430.c (msp430_file_end): Fix jumbled
	HAVE_AS_MSPABI_ATTRIBUTE and HAVE_AS_GNU_ATTRIBUTE checks.
	* configure: Regenerate.
	* configure.ac: Use ".mspabi_attribute 4,2" to check for assembler
	support for this object attribute directive.