ARM: add @llvm.arm.space intrinsic for testing ConstantIslands.

ARM: add @llvm.arm.space intrinsic for testing ConstantIslands.

Creating tests for the ConstantIslands pass is very difficult, since it depends
on precise layout details. Having the ability to precisely inject a number of
bytes into the stream helps greatly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221903 91177308-0d34-0410-b5e6-96231b3b80d8