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/.