unwind: use sizeof() instead of hardcoded sizes

The statically allocated strings have a fixed size which can be computed using
the sizeof operator rather than duplicating the allocation size which can drift.
NFC.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@229126 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed