Generating unistd_*.h

["H. Peter Anvin" <hpa@zytor.com>]

I am working on a patchset after which unistd_*.h in the x86 tree will
be autogenerated by scripts.  This is a substantial cleanup and a
preparation for the x32 setup.

However, one consequence of this is that <asm/unistd.h> will now include
a number of autogenerated fragments, some of which will be needed by
"make headers-install".  Furthermore, since <asm/asm-offsets.h> depends
on <asm/unistd.h> this creates a new dependency which I'm not sure what
the best way to handle is from a kbuild perspective.

1. Is there a better place than others to put these generated files
   (i.e. other than $(obj)/arch/x86/include/asm)?

2. Is there a better place than others to put the rules for generate
   these files?

3. There needs to be an arch-specific way to encode the dependency
   before asm-offsets.h is generated.  Preferences here?

	-hpa