ARM: linux-user: Expose VFP registers to signal handlers

For ARM linux-user mode signal handlers, fill in the ucontext with
VFP register contents in the same way that the kernel does. We only
do this for v2 format sigframe (2.6.12 and above); this is actually
bug-for-bug compatible with the older kernels, which don't save and
restore VFP registers either.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@nokia.com>
