graphics/opendx: Fix arch name

It's aarch64 not arm64.
While here:
  use BROKEN_aarch64 instead of NOT_FOR_ARCHS (same as the other ports with the same problem)
  add the error message

Approved by:	portmgr (Trivial fixes blanket)

graphics/opendx/Makefile

@@ -15,8 +15,7 @@ LICENSE_NAME=	IBM PUBLIC LICENSE - Open Visualization Data Explorer VERSION 1.0
 LICENSE_FILE=	${WRKSRC}/LICENSE
 LICENSE_PERMS=	dist-mirror dist-sell pkg-mirror pkg-sell auto-accept
 
-NOT_FOR_ARCHS=	arm64
-NOT_FOR_ARCHS_REASON=	does not build
+BROKEN_aarch64=	fails to link: undefined symbol: sbrk
 
 LIB_DEPENDS=	libtiff.so:graphics/tiff		\
 		libcdf.so:science/cdf