[arm] Mark -marm and -mthumb as being inverse options

-marm and -mthumb are opposites: one cancels out the other.  This patch
marks them as such so that the driver will eliminate all but the last
option on the command line.  This aids multilib selection which otherwise
can get confused if both are present.

	* config/arm/arm.opt (marm): Mark as the negative of of -mthumb.
	(mthumb): Mark as the negative of -marm.

From-SVN: r249309