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When calling newCAF, refer to the closure using its LocalReg rather
than R1.  Using R1 here was preventing the register allocator from
coalescing the assignment x=R1 at the beginning of the function.

aargh.

This saves compile time and can make a big difference in some
pathological cases (T4801)

This means that we now generate the same code whatever platform we are
on, which should help avoid changes on one platform breaking the build
on another.

It's also another step towards full cross-compilation.

To explicitly choose whether you want an unregisterised build you now
need to use the "--enable-unregisterised"/"--disable-unregisterised"
configure flags.

This is a bit odd by itself, but it's a stepping stone on the way to
putting "target unregisterised" into the settings file.

No functional differences yet

We weren't defining it in the other places that MachRegs.h gets
imported, which seems a little suspicious. And if it's not defined
then it defaults to 4 anyway, so this define doesn't seem necessary.

The new code generator now apparently generates a working stage2
compiler.

See Note [foreign calls clobber GlobalRegs]

Related to #4862

All uses of it are now in an IO Monad, so we don't need to use
a trace-like function.

It's no longer a trace-like function.

They were getting baked into Config.hs before.