Make libcxxrt's parsing of DWARF exception handling tables work on
architectures with strict alignment, by using memcpy() instead of
directly reading fields.

Reported by: Daisuke Aoyama <aoyama@peach.ne.jp>
Reviewed by: imp, bapt
Tested by: bapt
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D1967