Adds a builtin and intrinsic for the f32.store_f16 instruction.
The instruction stores an f32 value as an f16 memory. Specified at:
29a9b9462c/proposals/half-precision/Overview.md
Note: the current spec has f32.store_f16 as opcode 0xFD0121, but this is
incorrect and will be changed to 0xFC31 soon.