check for C11 header stdalign.h

git-svn-id: https://svn.r-project.org/R/trunk@66908 00db46b3-68df-0310-9c12-caf00c1e9a41
use C11 features to find alignment

git-svn-id: https://svn.r-project.org/R/trunk@66909 00db46b3-68df-0310-9c12-caf00c1e9a41