Add the C23 memset_explicit function to glibc.  Everything here is
closely based on the approach taken for explicit_bzero.  This includes
the bits that relate to internal uses of explicit_bzero within glibc
(although we don't currently have any such internal uses of
memset_explicit), and also includes the nonnull attribute (when we
move to nonnull_if_nonzero for various functions following C2y, this
function should be included in that change).

The function is declared both for __USE_MISC and for __GLIBC_USE (ISOC23)
(so by default not just for compilers defaulting to C23 mode).

Tested for x86_64 and x86.