MIPS: Add FP_MODE regset support

Define an NT_MIPS_FP_MODE core file note and implement a corresponding
regset holding the state handled by PR_SET_FP_MODE and PR_GET_FP_MODE
prctl(2) requests.  This lets debug software correctly interpret the
contents of floating-point general registers both in live debugging and
in core files, and also switch floating-point modes of a live process.

[paul.burton@mips.com:
  - Changed NT_MIPS_FP_MODE to 0x801 to match first nibble of
    NT_MIPS_DSP, which was also changed to avoid a conflict.]

Signed-off-by: Maciej W. Rozycki <macro@mips.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/19331/
Cc: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org