xtensa: enable CORE_DUMP_USE_REGSET

Drop xtensa_elf_core_copy_regs function, ELF_CORE_COPY_REGS macro, and
dump_fpu function. Define CORE_DUMP_USE_REGSET to make ELF core dumper
use regset interface.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>