[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

gcc/ChangeLog

 2017-06-16  Richard Earnshaw  <rearnsha@arm.com>
 
+	* config/arm/arm.opt (marm): Mark as the negative of of -mthumb.
+	(mthumb): Mark as the negative of -marm.
+
+2017-06-16  Richard Earnshaw  <rearnsha@arm.com>
+
 	* doc/invoke.texi (ARM Options, -mcpu): Document supported
 	extension options.
 	(ARM Options, -mtune): Document that this accepts the same

gcc/config/arm/arm.opt

@@ -91,7 +91,7 @@ EnumValue
 Enum(arm_arch) String(native) Value(-1) DriverOnly
 
 marm
-Target Report RejectNegative InverseMask(THUMB)
+Target Report RejectNegative Negative(mthumb) InverseMask(THUMB)
 Generate code in 32 bit ARM state.
 
 mbig-endian
@@ -195,7 +195,7 @@ Target RejectNegative Joined UInteger Var(arm_structure_size_boundary) Init(DEFA
 Specify the minimum bit alignment of structures.
 
 mthumb
-Target Report RejectNegative Mask(THUMB) Save
+Target Report RejectNegative Negative(marm) Mask(THUMB) Save
 Generate code for Thumb state.
 
 mthumb-interwork