Add of_char{16,32}_t.

For ObjC++, these are defined to char{16,32}_t if C++11 is used. This
means passing e.g. a C++11 UTF-16 string to -[initWithUTF16String:]
works without a warning.

For C11, this is not necessary, as char{16,32}_t are just typedef'd to
uint_least{16,32}_t in <uchar.h> and thus compatible to the definition
of of_char{16,32}_t. The reason for not defining of_char{16,32}_t to
char{16,32}_t when using C11 is that <uchar.h> might be missing and that
char{16,32}_t are not required to be UTF-{16,32}, whereas in C++, they
are required to be UTF-{16,32}.