unwind: use explicit memcpy for register saving
unwind: use explicit memcpy for register saving

Convert the register saving code to use an explicit memcpy rather than the
implicit memcpy from the assignment.  This avoids warnings from -Wcast-qual on
GCC and makes the code more explicit.  Furthermore, use sizeof to calculate the
offsets rather than adding magic numbers, improving legibility of the code.
NFC.

git-svn-id: 6a9f6578bdee8d959f0ed58970538b4ab6004734@228904 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi