kbuild: Add rules for compiling programs on the host

At a couple of places, we need to build programs which run on the
compile host during building the kernel.

Add a new variable "host-progs" to declare such programs. Rules.make will
automatically provide rules to compile these programs with appropriate
command lines etc.

Compared to having explicit rules spread around for doing so, this has
the following advantages:

o shorter
o automatically figures out dependencies and handles changing command
  lines
o Nicer output in quiet mode ;-)

Compiling host programs from multiple sources is also possible, analogous
to multi-part objects. E.g. scripts/Makefile has

	host-progs := tkparse
	tkparse-objs := tkparse.o tkcond.o tkgen.o

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