Add new ELF note types from Linux 4.19 to elf.h

[Florian Weimer]

* Joseph Myers:

> +#define NT_MIPS_DSP	0x800		/* MIPS DSP ASE registers.  */
> +#define NT_MIPS_FP_MODE	0x801		/* MIPS floating-point mode.  */

Looks good to me.  Values match what's in the kernel sources.

Thanks,
Florian