MIPS: Don't compile math-emu when CONFIG_MIPS_FP_SUPPORT=n

When CONFIG_MIPS_FP_SUPPORT=n we don't support floating point, so
there's no point compiling in our FPU emulator. Avoid doing so,
providing stub versions of dsemul cleanup functions that are called from
signal & task handling code.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/21012/
Cc: linux-mips@linux-mips.org