cmd/compile: add 32 bit float registers/variables on wasm

cmd/compile: add 32 bit float registers/variables on wasm

Before this change, wasm only used float variables with a size of 64 bit
and applied rounding to 32 bit precision where necessary. This change
adds proper 32 bit float variables.

Reduces the size of pkg/js_wasm by 254 bytes.

Change-Id: Ieabe846a8cb283d66def3cdf11e2523b3b31f345
Reviewed-on: https://go-review.googlesource.com/c/go/+/195117
Reviewed-by: Cherry Zhang <cherryyz@google.com>