ARM: Don't unconditionally bloat thread_info

There is no point reserving space at the bottom of the kernel stack for
per-thread crunch state, and per-thread VFP state if these are not being
supported by the kernel being built.  Remove these members from the
thread union when these features are disabled.

Reported-by: Tim Bird <tim.bird@am.sony.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>