go/build: omit PkgObj for packages "unsafe" and "builtin"

go/build: omit PkgObj for packages "unsafe" and "builtin"

Package "builtin" is not a real, importable package; it exists only
for documentation. Package "unsafe" is not compiled into an object
file from its source code; instead, imports of "unsafe" are handled
specially by the compiler.

(In Go 1.19.3, package "unsafe" did not have an install target, while
package "builtin" did but that target was never written.)

Fixes #56687.
Updates #47257.

Change-Id: I1d1e90ff9e1629b80e0df93e1f7e17242c8dab69
Reviewed-on: https://go-review.googlesource.com/c/go/+/449376
Reviewed-by: Michael Matloob <matloob@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>