cmd/compile: 1.15 heap allocations regression when calling Write on os.Stdout [1.15 backport]

@ianlancetaylor requested issue #41474 to be considered for backport to the next 1.15 minor release.

> @gopherbot Please open a backport issue for 1.15.
> 
> This is an unexpected and unavoidable performance regression in 1.15: using `Write` to write a `[]byte` forces the `[]byte` to escape into the heap.  That did not happen in 1.14.
