liquid-dsp: work-around compiler bug on SuperH

On SuperH, the BR2_PACKAGE_LIQUID_DSP_FAST option of the liquid-dsp
package causes triggers a compiler bug with the Sourcery CodeBench
toolchains:

  src/filter/src/firdes.c:406:1: internal compiler error: in gen_reg_rtx, at emit-rtl.c:859

This commit works around this problem by simply making this option
unavailable with the problematic toolchains.

Fixes:

  http://autobuild.buildroot.net/results/d43/d43574b8f487648bc440fbe0b63b2a08b309dfc7/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>