Use ISO_C_VISIBLE, as opposed to testing STDC_VERSION__.

FreeBSD's C library uses STDC_VERSION to determine whether the
compiler provides language features specific to a certain version of the
C standard. __ISO_C_VISIBLE is used to specify which library features
need to be exposed.

max_align_t currently uses STDC_VERSION, even though it should be
using __ISO_C_VISIBLE to remain consistent with the rest of the headers
in include/.

Reviewed by: dim
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D11303