[ARM] Disabling vfp4 should disable fp16

ARMTargetParser::getFPUFeatures should disable fp16 whenever it
disables vfp4, as otherwise something like -mcpu=cortex-a7 -mfpu=none
leaves us with fp16 enabled (though the only effect that will have is
a wrong build attribute).

Differential Revision: http://reviews.llvm.org/D10397

llvm-svn: 239599