go.bbclass: Add `-trimpath` to default build flags

The `-trimpath` option is important for reproducible builds so full
build paths and module paths are not embedded.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit e3df027b)
Signed-off-by: Steve Sakoman <steve@sakoman.com>