x86, boot: Restrict CFLAGS for hostprogs

Currently tools/build has access to all the kernel headers in
$(srctree). This is unnecessary and could potentially allow
tools/build to erroneously include kernel headers when it should only
be including userspace-exported headers.

Unfortunately, mkcpustr still needs access to some of the asm kernel
headers, so explicitly special case that hostprog.

Cc: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>
Link: http://lkml.kernel.org/r/1330436245-24875-5-git-send-email-matt@console-pimps.org
Signed-off-by: H. Peter Anvin <hpa@zytor.com>