arch/arm: remove BR2_GCC_TARGET_ARCH definitions on ARM

On ARM, we were defining both the CPU type and the architecture
variant. However, depending on the version of gcc, a given combination
of (CPU, architecture) may not be the same. Since the architecture
variant is implied by the CPU type, given the former is not necessary,
and we can simply specify the latter.

>From the gcc documentation:

  This specifies the name of the target ARM processor. GCC uses this
  name to derive the name of the target ARM architecture (as if
  specified by -march) and the ARM processor type for which to tune
  for performance (as if specified by -mtune). Where this option is
  used in conjunction with -march or -mtune, those options take
  precedence over the appropriate part of this option.

Note that we verified that for all BR2_GCC_TARGET_ARCH value that
existed, a proper BR2_GCC_TARGET_CPU value is defined.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>