re-add ELF gregs and fpregs types to riscv64 user.h

d493206d deleted all the content of
user.h, but sys/procfs.h expects this from sys/user.h
threfore we retain the non conflicting parts

arch/riscv64/bits/user.h

+struct user_fpregs_struct {
+	double f[32];
+	unsigned int fcsr;
+};
+
+#define ELF_NGREG 32
+typedef unsigned long elf_greg_t, elf_gregset_t[ELF_NGREG];
+typedef struct user_fpregs_struct elf_fpregset_t;