fix riscv64 elf_fpregset_t type and member names mismatch

[musl] / arch / riscv64 / bits / user.h

#include <signal.h>

#define ELF_NGREG 32
typedef unsigned long elf_greg_t, elf_gregset_t[ELF_NGREG];
typedef union __riscv_mc_fp_state elf_fpregset_t;