Kernel/riscv64: Implement {load,store}_fpu_state

This commit adds two functions which save/restore the entire FPU state.
On RISC-V, you only need to save the floating pointer registers
themselves and the fcsr CSR, which contains the entire state of the F/D
extensions.

Kernel/Arch/riscv64/FPUState.h

@@ -16,6 +16,7 @@ namespace Kernel {
 // This struct will get pushed on the stack by the signal handling code.
 // Therefore, it has to be aligned to a 16-byte boundary.
 struct [[gnu::aligned(16)]] FPUState {
+    // FIXME: Add support for the Q extension.
     u64 f[32];
     u64 fcsr;
 };