arch: arm: aarch32: cortex_a_r: Add shared FPU support

This adds lazy floating point context switching.  On svc/irq entrance,
the VFP is disabled and a pointer to the exception stack frame is saved
away.  If the esf pointer is still valid on exception exit, then no
other context used the VFP so the context is still valid and nothing
needs to be restored.  If the esf pointer is NULL on exception exit,
then some other context used the VFP and the floating point context is
restored from the esf.

The undefined instruction handler is responsible for saving away the
floating point context if needed.  If the handler is in the first
irq/svc context and the current thread uses the VFP, then the float
context needs to be saved.  Also, if the handler is in a nested context
and the previous context was using the FVP, save the float context.

Signed-off-by: Bradley Bolen <bbolen@lexmark.com>