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.

llvm-svn: 228904