cmd/go, cmd/compile: always optimize when building runtime
When optimizations are disabled, the compiler
cannot eliminate enough write barriers to satisfy
the runtime's nowritebarrier and nowritebarrierrec
annotations.
Enforce that requirement, and for convenience,
have cmd/go elide -N when compiling the runtime.
This came up in practice for me when running
toolstash -cmp. When toolstash -cmp detected
mismatches, it recompiled with -N, which caused
runtime compilation failures.