This splits mk/stageN.mk into host.mk and target.mk and makes
the build rules somewhat simpler - there's no more building from stageN
into stageN+1; instead we always build from stageN(host) to
stageN(target) then promote from stageN(target) to stageN+1(host).
Add a big honkin explaination right at the top of Makefile.in
Each stage is organized more according to Unix standards and to
accommodate multiple target architectures.
stageN/
bin - rustc lives here
lib - libraries that rustc needs
lib/rustc/$(target_triple/ - target libraries
This essentially starts the bootstrapping one step earlier by building
the stdlib from source using the stage0 compiler and then using that
stdlib to build the stage1 compiler. (Instead of starting by building
the stage1 compiler and then building a stdlib with it).
This means we should now be able to add features to the stdlib and use
them in the compiler without having to do a snapshot. (On the flip
side, this means that we now need to do a snapshot if we want to use a
new language feature in the stdlib, but that doesn't really seem too
burdensome (we already need to snapshot if we want to use a new
language feature in the compiler)).
