fix riscv64 elf_fpregset_t type and member names mismatch

fix riscv64 elf_fpregset_t type and member names mismatch

in order for sys/procfs.h (provided by sys/user.h) to be useful, it
needs to match the API its consumers (gdb, etc.) expect, including the
member names established by glibc.

this partly reverts commit 29e8737f81ccc9fbadcf61a75318aa3d0516aafa,
which partly reverted d493206de7df4db07ad34f24701539ba0a6ed38c,
eliminating struct user_fpregs_struct which seems to have had no
precedent and using union __riscv_mc_fp_state for elf_fpregset_t. this
requires indirect inclusion of signal.h to make union
__riscv_mc_fp_state visible, but being that these are nonstandard
"junk" headers with no official restrictions on what they can pull in,
that's no big deal.

split off and expanded from patch by Khem Raj.