This is necessary for the following:
- to make sure os/exec can be imported
- to make sure internal/testenv can be imported
The internal/testenv package (which imports os/exec) is used by a lot of
tests. By adding support for it, more tests can be run.
This commit adds a bunch of new packages that now pass all tests.
So far, we've pretended to be js/wasm in baremetal targets to make the
stdlib happy. Unfortunately, this has various problems because
syscall/js (a dependency of many stdlib packages) thinks it can do JS
calls, and emulating them gets quite hard with all changes to the
syscall/js packages in Go 1.12.
This commit does a few things:
* It lets baremetal targets pretend to be linux/arm instead of
js/wasm.
* It lets the loader only select particular packages from the src
overlay, instead of inserting them just before GOROOT. This makes it
possible to pick which packages to overlay for a given target.
* It adds a baremetal-only syscall package that stubs out almost all
syscalls.
