libstdc++: Fix _Hash_bytes for I16LP32 targets [PR107885]

For H8/300 size_t is 32 bits wide, but (unsigned char)buf[2] << 16
promotes to int which is only 16 bits wide. The shift is then undefined.
This fixes it by converting to size_t before shifting.

libstdc++-v3/ChangeLog:

	PR libstdc++/107885
	* libsupc++/hash_bytes.cc (_Hash_bytes): Convert to size_t
	instead of implicit integer promotion to 16 bits.