qt5: disallow 5.8 version for ARMv4

Fixes:
http://autobuild.buildroot.net/results/197/197ab8a12bf1d780a986e5d447d1e0f5ee070405/



qt5.8 no longer supports ARMv4. From src/corelib/global/qprocessordetection.h:

  if Q_PROCESSOR_ARM >= 6
    define Q_PROCESSOR_ARM_V6
  endif
  if Q_PROCESSOR_ARM >= 5
    define Q_PROCESSOR_ARM_V5
  else
    error "ARM architecture too old"

[Peter: also hide comment as pointed out by Baruch]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>