unwind: clean up -Wundef warnings

The unified register management interfaces had multiple naked macros for
conditional logic.  This cleans them up to use the defined() form, avoiding
-Wundef warnings.  NFC.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@228663 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed