Add XCore support for ATOMIC_FENCE.

ATOMIC_FENCE is lowered to a compiler barrier which is codegen only. There
is no need to emit an instructions since the XCore provides sequential
consistency.

Original patch by Richard Osborne

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194464 91177308-0d34-0410-b5e6-96231b3b80d8
XCore target test for hidden declaration

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194465 91177308-0d34-0410-b5e6-96231b3b80d8
XCore target: fix bug in aligning 'byval i8*' on the stack

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194466 91177308-0d34-0410-b5e6-96231b3b80d8