internal/sys: add LR and fixed frame size to sys.Arch
Storing this information in the Arch eliminates some code duplication
between the compiler and linker. This information is entirely
determined by the Arch, so the current approach of attaching it to an
entire Ctxt is a little silly. This will also make it easier to use
this information from tests.
The next CL will be a rote refactoring to eliminate the
Ctxt.FixedFrameSize methods.