ARM: VFP: preserve the HW context when calling signal handlers

BugLink: http://bugs.launchpad.net/bugs/507503

Signal handlers can use floating point, so prevent them to corrupt
the main thread's VFP context. So far there were two signal stack
frame formats defined based on the VFP implementation, but the user
struct used for ptrace covers all posibilities, so use it for the
signal stack too. This patch extends the user struct and leaves
its magic number the same, in the hope that user space code does
not depend on its size and can parse the original regs w/o
problems.

Support to save / restore the exception registers was added by
Will Deacon.

Signed-off-by: Imre Deak <imre.deak@nokia.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Bryan Wu <bryan.wu@canonical.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>