RISC-V: Implement option to run NuttX in supervisor mode (S-mode)

- Add config "ARCH_USE_S_MODE" which controls whether the kernel
  runs in M-mode or S-mode
- Add more MSTATUS and most of the SSTATUS register definitions
- Add more MIP flags for interrupt delegation
- Add handling of interrupts from S-mode
- Add handling of FPU from S-mode
- Add new context handling functions that are not dependent on the trap
  handlers / ecall

NOTE: S-mode requires a companion SW (SBI) which is not yet implemented,
      thus S-mode is not usable as is, yet.

boards/risc-v/c906/smartl-c906/src/c906_ostest.c

@@ -31,6 +31,8 @@
 
 #include <nuttx/irq.h>
 
+#include "riscv_internal.h"
+
 /****************************************************************************
  * Pre-processor Definitions
  ****************************************************************************/