[dev.regabi] cmd/compile: cleanup //go:generate directives

During recent refactoring, we moved mkbuiltin.go to package typecheck,
but accidentally duplicated its //go:generate directive into a bunch
of other files/directories. This CL cleans up the unnecessary
duplicates.

Also, update all of the stringer invocations to use an explicit file
name, and regenerate their files.

Updates #43369.

Change-Id: I4e493c1fff103d742de0a839d7a3375659270b50
Reviewed-on: https://go-review.googlesource.com/c/go/+/280635


Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Meng Zhuo <mzh@golangcn.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Meng Zhuo <mzh@golangcn.org>