Support darwin/linux specific go sources

In order to use certain syscalls, some source must only be compiled on
one of darwin or linux. There are also cases where it's simplest to have
two implementations of a method (or struct) instead of choosing an
implementation at runtime.

The standard go tools use _<GOOS>.go for this, but the explicit structs
map better into the current blueprint format (and you can still use
_darwin.go / _linux.go to maintain compatiblity with those tools).

Test: Define _linux.go / _darwin.go files, ensure only one is compiled.
Change-Id: I58bffefbf839ba52dea3d8e7d3ec67eaa721c463