Fix test case in r211605/r211533

The test case in
"Fix PR20056: Implement pseudo LDR <reg>, =<literal/label> for AArch64" should
only work with Linux.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211613 91177308-0d34-0410-b5e6-96231b3b80d8