cmd/compile: don't bother compiling functions named "_"

They can't be used, so we don't need code generated for them. We just
need to report errors in their bodies.

The compiler currently has a bunch of special cases sprinkled about
for "_" functions, because we never generate a linker symbol for them.
Instead, abort compilation earlier so we never reach any of that
special-case code.

Fixes #29870

Change-Id: I3530c9c353deabcf75ce9072c0b740e992349ee5
Reviewed-on: https://go-review.googlesource.com/c/158845


Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>