MIPS: Correct MIPS I FP context layout

MIPS: Correct MIPS I FP context layout

Implement the correct ordering of individual floating-point registers
within double-precision register pairs for the MIPS I FP context, as
required by our FP emulation code and expected by userland talking via
ptrace(2).  Use L.D and S.D assembly macros that do the right thing like
LDC1 and SDC1 from MIPS II up, avoiding the need to mess up with
endianness conditionals.

This in particular fixes the handling of denormals and NaN generation in
Unimplemented Operation emulation traps.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9699/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>