[ARM, VXworks] Fix build

[ARM, VXworks] Fix build

My patch last week to address selection of be8 linking mode broke the
build for vxworks.  It turns out that this port is one of the few
remaining that is still not based on the EABI/AAPCS.

This patch fixes the build, but I've not really tested it beyond
building the core compiler binaries.  Building a workable compiler
entails downloading a load of vxworks stuff that I'm not sure where to
find.

	* config/arm/vxworks.h (TARGET_ENDIAN_DEFAULT): Define.

From-SVN: r250177