Match _Nonnull between char and wchar_t funcs.

The char versions of these functions have `_Nonnull` on these
parameters. Match it in the wide char versions. strxfrm_l also has
`__restrict` on its pointer arguments, I believe we've decided that's
something that shouldn't be proliferated...

Test: make checkbuild tests
Bug: None
Change-Id: Ie533e1af92b2a7c9df657073dbc4c345abf72473