Add more context information to the stop-hook mechanism by displaying the stop-hook

command in the '- Hook id' header.  This should improve readbility of the 'display'
command if, for example, we have issued 'display a' and 'display b' which turn into
"target stop-hook add -o 'expr -- a'" and "target stop-hook add -o 'expr -- b'".

Plus some minor change in TestAbbreviations.py to conditionalize the platform-specific
checkings of the "image list" output.

llvm-svn: 142868