ARC: Implement native GDB requirements in procfs.h

Native GDB requires elf_fpregset_t to be defined, even if floating point
registers are absent. In this case we use empty structure. Additionally we
redefined prfpregset_t to use this new type.

And there has been an error: it was defined that user_regs_struct has 20
registers, while it has 40.

Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>