[msan] Change va_start/va_copy shadow memset alignment to 8.

This fixes va_start/va_copy of a va_list field which happens to not
be laid out at a 16-byte boundary.

Differential Revision: http://llvm-reviews.chandlerc.com/D276

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172128 91177308-0d34-0410-b5e6-96231b3b80d8