The previous checkstack fix for UML, which needs to use the host's tools,
was wrong in the crossbuilding case.  It would use the build host's, rather
than the target's, toolchain.

This patch removes the old fix and adds an explicit special case for UML,
leaving everyone else alone.
Signed-off-by: NJeff Dike <jdike@addtoit.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>