riscv64: define ELF_NFPREG

ELF_NFPREG is used by some userspace applications like gdb

arch/riscv64/bits/user.h

@@ -1,5 +1,6 @@
 #include <signal.h>
 
 #define ELF_NGREG 32
+#define ELF_NFPREG 33
 typedef unsigned long elf_greg_t, elf_gregset_t[ELF_NGREG];
 typedef union __riscv_mc_fp_state elf_fpregset_t;