Remove skip and xfail decorators for target architecture.

This removes the following decorators:

* skipIfI386
* expectedFailureI386
* expectedFailurex86_64
* skipIfArch
* skipUnlessArch
* skipUnlessI386

And other related decorators.  All code using those decorators
is updated to use expectedFailureAll and skipIf

llvm-svn: 260178