Preserve VFP state across signal delivery.
rS315974
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Description

Preserve VFP state across signal delivery.

We don't have enouch space to store full VFP context within mcontext
stucture. Due to this:

follow i386/amd64 way and store VFP state outside of the mcontext_t but point to it. Use the size of VFP state structure as an 'magic' indicator of the saved VFP state presence.
teach set_mcontext() about this external storage.
for signal delivery, store VFP state to expanded 'struct sigframe'.

Submited by: Andrew Gierth (initial version)
PR: 217611
MFC after: 2 weeks