Currently, the empty function bodies actually contain the byte 0, which
is the unreachable opcode. This CL fixes this to be empty function
bodies, and uses the macros more consistently.

This is the sixth CL in a series to improve our module decoder tests and
make them more readable.

R=titzer@chromium.org

Bug: v8:8238
Change-Id: I5f029210b4589797ee194e4082afec2c7bc31561
Reviewed-on: https://chromium-review.googlesource.com/c/1286343Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56736}