Makefile: remove 'mythical' build flags

1) The root's and the arch's Makefile do not both need to
contain the same build flags. The root's Makefile inherits
the arch's flags.

2) It is unnecessary to define the -mfpu=X flag, since the
arch's Makefile sets -msoft-float, which excludes the usage
of hardware vfp units.

3) Automatic vectorization via -ftree-vectorize does not
work with -msoft-float. Even if it were working, automatic
vectorization does not give noteworthy improvements in
performance.

4) In arch/arm/vfp/Makefile, it is counter-productive to
change the -mfpu=X assembler flag, since there's no hardware
vfp usable, as pointed out before.

Information sources:
  https://wiki.debian.org/ArmHardFloatPort/VfpComparison

  https://lkml.org/lkml/2011/10/3/254