Remove old/broken PPC/Altivec code.

* Removes FLAC__lpc_restore_signal_asm_ppc_altivec_16*
  from lpc.h and stream_decoder.c
* Removes PPC-specific code from cpu.c and cpu.h
* Removes PPC stuff from libFLAC/Makefile.lite and build/*.mk
* Removes as/gas/PPC-specific stuff from configure.ac and
  libFLAC/Makefile.am*
* Removes libFLAC/ppc folder and remove "src/libFLAC/ppc*/Makefile"
  lines from configure.ac

Patch-from: lvqcl <lvqcl.mail@gmail.com>