This commit does two things:
* It adds support for the GOOS and GOARCH environment variables. They
fall back to runtime.GO* only when not available.
* It adds support for 3 new architectures: 386, arm, and arm64. For
now, this is Linux-only.
This avoids a ton of duplication and makes it easier to change a generic
target (for example, the "cortex-m" target) for all boards that use it.
Also, by making it possible to inherit properties from a parent target
specification, it is easier to support out-of-tree boards that don't
have to be updated so often. A target specification for a
special-purpose board can simply inherit the specification of a
supported chip and override the properites it needs to override (like
the programming interface).
