ARM: Fix instruction set selection for GCC 11

GCC 11 on ARM now complains like the following when trying to determine if
an arch is supported. Presumably because it enforces the default option
'--with-float=hard' which GCC 10 didn't do?

  $ arm-linux-gnueabihf-gcc-11 -march=armv7-a -c -x c /dev/null
  cc1: error: ‘-mfloat-abi=hard’: selected architecture lacks an FPU

Due to that, the kernel build system selects the wrong compiler options
which throws errros like this during the build:

  /tmp/ccrHfZPj.s: Assembler messages:
  /tmp/ccrHfZPj.s:116: Error: selected processor does not support `dmb ish' in ARM mode
  /tmp/ccrHfZPj.s:150: Error: selected processor does not support `isb ' in ARM mode
  /tmp/ccrHfZPj.s:160: Error: selected processor does not support `mrrc p15,1,r4,r5,c14' in ARM mode
  /tmp/ccrHfZPj.s:245: Error: selected processor does not support `dmb ish' in ARM mode
  /tmp/ccrHfZPj.s:503: Error: selected processor does not support `dmb ish' in ARM mode
  /tmp/ccrHfZPj.s:527: Error: selected processor does not support `dmb ish' in ARM mode
  /tmp/ccrHfZPj.s:698: Error: selected processor does not support `dmb ish' in ARM mode
  /tmp/ccrHfZPj.s:731: Error: selected processor does not support `isb ' in ARM mode

Fix that by adding '-msoft-float' to KBUILD_CFLAGS before the definition of
the 'arch-$(CONFIG_CPU_<foo>)' instruction selection macros.

Signed-off-by: Juerg Haefliger <juergh@canonical.com>