[x86] Teach the test update script to strip trailing whitespace.

[x86] Teach the test update script to strip trailing whitespace.

This is done in a bit of a strange way to use a multiline RE instead of
looping over the lines. Suggestions welcome here for a more pythonic way
of doing this as long as its reasonably fast.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228131 91177308-0d34-0410-b5e6-96231b3b80d8