[arch][riscv] add fpu context switch support

Currently only implemented for double precision floating point.

Caveat: currently unable to only compile some code with or without
float. The linker is extremely picky about mixing float and no-float
objects, so stick with all on or off for now.

It's not as much of a problem currently because the toolchain is not
using any riscv vector instructions to assist normal code, so it's
generally only emitting fpu instructions for floating point code.

app/tests/float.c

@@ -5,7 +5,7 @@
  * license that can be found in the LICENSE file or at
  * https://opensource.org/licenses/MIT
  */
-#if ARM_WITH_VFP || ARCH_ARM64 || X86_WITH_FPU
+#if ARM_WITH_VFP || ARCH_ARM64 || X86_WITH_FPU || (ARCH_RISCV && RISCV_FPU)
 
 #include <stdio.h>
 #include <rand.h>