This reverts commit 371a316e.

Closes #7217

Apparently the standard --build and --host flags don't actually
_do_ anything. This re-uses the libuv flags, since they are the
same for getting jemalloc to cross-compile

This lets us use #ifdefs to determine which stage of the build we happen
to be in, which is useful in the event we need to make changes to rustrt
that are incompatible with the code generated by stage0.

This should help pave the way to completing #6575, which will likely
require changes to type signatures for spawn_fn & glue_fn in rustrt.

Closes #6142

For cross compiling to targets that don't want to build a compiler

- thanks to work in libuv's upstream, we can call libuv's Makefile directly
with parameters, instead of descending in gyp-uv madness and generating
our own.