fix aliasing-based undefined behavior in mbsrtowcs

fix aliasing-based undefined behavior in mbsrtowcs

mbsrtowcs contains "vectorized" loops to quickly step over bytes
without the high bit set; these have undefined behavior by virtue of
aliasing uint32_t over top of char data for the accesses.

commit 4d0a82170a25464c39522d7190b9fe302045ddb2 fixed the
corresponding usage in string functions by using the may_alias
attribute conditional on __GNUC__ and disabled the vectorized code in
its absence. do the same for mbsrtowcs.