ARM: correct handling of .fpu directive

The FPU directive permits the user to switch the target FPU, enabling
instructions that would be otherwise unavailable.  However, when configuring the
new subtarget features, we would not enable the implied functions for newer
FPUs.  This would result in invalid rejection of valid input.  Ensure that we
inherit the implied FPU functionality when enabling newer versions of the FPU.
Fortunately, these are mostly hierarchical, unlike the CPUs.

Addresses PR22395.

llvm-svn: 227584