Rework makefile for cross-compilation with a toolchain again.

Rework makefile for cross-compilation with a toolchain again.

We use Makefile.tools only if --enable-cross-compiling is true.
Everything it builds goes under tools/, or should if there are no
bugs -- even intermediate .bin, .com, &c., files.

New usage model does not require any external trees:

% ./configure --enable-cross-compiling --enable-native-code=i386
% make cross-host

% make all

However, the host must be a recent master with new CREF and SF
features for separate source and object roots, so we can't rely on
this to work until we cut a new release with those features.

(It shouldn't be necessary to run _anything_ on the target system,
but we're a ways away from that: we need the cross-compiler to be
able to cope with macros across subsystem dependencies, and we need a
`static' linker, not just a fasloader-and-disk-save.)