src/cmd/internal/obj/wasm: optimize blocks in wasm binary

This change optimizes the blocks in the wasm binary by generating the
entryPointLoop only if it is used and adding an unwindExit block to
be able to use the short BrIf instruction for unwinding the stack.
These changes were suggested by the wasm-opt tool and reduce the
wasm binary size of "hello world" by 1.5%.

Change-Id: Ie52db2fa2d9b8482f9a78b7c189231750811fe97
Reviewed-on: https://go-review.googlesource.com/c/go/+/167937


Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>