There's no point to having them in every GOOS_GOARCH directory,
since they are neither GOOS- nor GOARCH-specific.
(There used to be other headers that were.)

This makes building for additional toolchains easier:
no need to run make.bash at all.

Fixes #10049.

Change-Id: I710ecaafd7a5c8cad85ccd595ea9cb6058f553b3
Reviewed-on: https://go-review.googlesource.com/6471Reviewed-by: Rob Pike <r@golang.org>