boot: remove sparc, powerpc, ia64 code and references

While I'm here, remove the practically blank README.  Apparently this was
on Matt's "To-Do" list, but that was 4 years ago so I don't think it's
going to get done.
Really set default binutils to version 2.24

The last update of binutils missed changing the default in bsd.cpu.mk,
which means all the bootstrap and other tools were built with binutils
2.22 instead of 2.24.  This oversight is rectified now.
tip(1): Fix various warnings and errors emitted by clang

The most serious were void/return mismatches, but clang also complained
about ambiguous brackets and mis-matching printf formats.  It also didn't
like one particular K&R-formatted function, but I'm not sure why not.