MFC r279307:

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
Differential Revision: https://reviews.freebsd.org/D1967

MFC r279310:

Since newer versions of compiler-rt require unwind.h, and we want to use
the copy in libcxxrt for it, fix the arm-specific header to define the
_Unwind_Action type.

Submitted by: andrew