volk: fixup QA's fcompare to test all differences

This addresses bug 627 by adding a fabs call when checking if a number
is close to 0. If this is the case we also make sure the other buffer
is within tolerance. For the normal test case wrap the percent difference
in fabs rather than just the difference.
volk: Fix the volk_32fc_s32fc_x2_rotator_32fc_generic kernel

abs() is an int function ...

Call the right one depending on the language now.

Note that I explicitely did the selection here rather than add a lv_abs()
function in volk_complex.h because there is no GCC extension for abs() that
would return the right type, so I'd have to stick to the cabs() function
which would force a double cast back and forth.

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>