arm64: elf: fix core dumping definitions for GP and FP registers

struct user_fp does not exist for arm64, so use struct user_fpsimd_state
instead for the ELF core dumping definitions. Furthermore, since we use
regset-based core dumping, we do not need definitions for dump_task_regs
and dump_fpu.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>