riscv: improve contended FPU switching

We can leverage the FPU dirty state as an indicator for preemptively
reloading the FPU content when a thread that did use the FPU before
being scheduled out is scheduled back in. This avoids the FPU access
trap overhead when switching between multiple threads with heavy FPU
usage.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>