[WebAssembly] Implement prototype f32.store_f16 instruction. (#91545)

Adds a builtin and intrinsic for the f32.store_f16 instruction.

The instruction stores an f32 value as an f16 memory. Specified at:

https://github.com/WebAssembly/half-precision/blob/29a9b9462c9285d4ccc1a5dc39214ddfd1892658/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.