title: enable fpu support for aarch64 (armv8-a) on gcc version 5.0.1
body: <p>I am measuring the GFLOPS performance of the Cortex-a57 with the HPLinpack benchmarks and it barely achieves 1 FP/cycle (considering ~2.4 GFLOPS @ 2.4 GHz). Since the old compiler (gcc 4.9.1) complained with several version of the -mfpu= option, I tried to configure gcc 5.0.1 as below</p>
  
  <p>../gcc/configure --with-gmp=/tmp/gcc --with-mpfr=/tmp/gcc --with-mpc=/tmp/gcc --with-libelf=/tmp/gcc --enable-languages=c,c++,fortran,go --target=aarch64-linux-gnu --prefix=/opt/another-gcc5 --with-arch=armv8-a --with-cpu=cortex-a57 --with-fpu=neon-fp-armv8</p>
  
  <p>configuration went fine, but when invoking make the --with-fpu flag crashed in gcc/gcc/config.gcc :4351</p>
  
  <p>echo "This target does not support --with-$option." 2>&amp;1</p>
  
  <p>due to the supported_defaults for the aarch64 architecture as defined in gcc/gcc/config.gcc 3464:3467</p>
  
  <p>supported_defaults=
    case "${target}" in
      aarch64*-<em>-</em>)
          supported_defaults="abi cpu arch"</p>
  
  <p>How come the fpu option is not supported? any advice is appreciated, I have never done this before and I am a bit lost :)</p>