avcodec/ppc/fdctdsp: POWER LE support in ff_fdct_altivec() delete macros VEC_FMERGEH() VEC_FMERGEL(), they where wrong

GCC tool had a bug of PPC intrinsic interpret, which has been fixed in GCC 4.9.1. This bug lead to
errors in two of our previous patches. We found this when we update our GCC tools to 4.9.1 and by
reading the related info on GCC website. We fix our previous error in two separate commits
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>