Add "-modd-spreg" when compiling the kernel for mips32r6 target.

This makes sure the kernel builds properly even with toolchains that
use "-mno-odd-spreg" by default. This is the case with Android gcc.
Prior to this patch, kernel builds using gcc for Android failed with
following error messages, if target architecture is set to mips32r6:

arch/mips/kernel/r4k_switch.S: Assembler messages:
.../r4k_switch.S:210: Error: float register should be even, was 1
.../r4k_switch.S:212: Error: float register should be even, was 3
.../r4k_switch.S:214: Error: float register should be even, was 5
.../r4k_switch.S:216: Error: float register should be even, was 7
.../r4k_switch.S:218: Error: float register should be even, was 9
.../r4k_switch.S:220: Error: float register should be even, was 11
.../r4k_switch.S:222: Error: float register should be even, was 13
.../r4k_switch.S:224: Error: float register should be even, was 15
.../r4k_switch.S:226: Error: float register should be even, was 17
.../r4k_switch.S:228: Error: float register should be even, was 19
.../r4k_switch.S:230: Error: float register should be even, was 21
.../r4k_switch.S:232: Error: float register should be even, was 23
.../r4k_switch.S:234: Error: float register should be even, was 25
.../r4k_switch.S:236: Error: float register should be even, was 27
.../r4k_switch.S:238: Error: float register should be even, was 29
.../r4k_switch.S:240: Error: float register should be even, was 31
make[2]: *** [arch/mips/kernel/r4k_switch.o] Error 1
Signed-off-by: Miodrag Dinic <miodrag.dinic@imgtec.com>
Signed-off-by: Goran Ferenc <goran.ferenc@imgtec.com>
Signed-off-by: Aleksandar Markovic <aleksandar.markovic@imgtec.com>
Cc: James.Hogan@imgtec.com
Cc: Paul.Burton@imgtec.com
Cc: Raghu.Gandham@imgtec.com
Cc: Leonid.Yegoshin@imgtec.com
Cc: Douglas.Leung@imgtec.com
Cc: Petar.Jovanovic@imgtec.com
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16509/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>