Actually, this looks like the same error that Judah Jacobson encountered. Try removing ghc/includes/ghcplatform.h, re-make in ghc/includes, and continue with the build.
Indeed, that helped to get a bit further, but after doing make clean;make I now get ../../ghc/compiler/ghc-inplace -H16m -O -fglasgow-exts -cpp -Iinclude -"#include" HsBase.h -funbox-strict-fields -ignore-package base -O -Rghc-timing -fgenerics -fgenerics -split-objs -c GHC/Float.lhs -o GHC/Float.o -ohi GHC/Float.hi /tmp/ghc21284.split__33.s:308:Fixup of 486 is not to a 4 byte address ******************** <<ghc: 737285752 bytes, 124 GCs, 4804665/8325060 avg/max bytes residency (8 samples), 23M in use, 0.01 INIT (0.00 elapsed), 4.26 MUT (15.08 elapsed), 1.50 GC (1.58 elapsed) :ghc>> make[2]: *** [GHC/Float.o] Error 1 make[1]: *** [all] Error 1 make: *** [build] Error 1 Johan Glimming.