[ARM] Allow setting the generic features for native cpus

Copying the already existing code for x86 to ARM to set the correct CPU
features when using -mcpu=native. We can already detect the CPU name
but we were not setting the correct feature bits.

Moving fpu/hwdiv down to make sure they override whatever we set the
default to be.

No tests because this is native detection, and not all ARM-enabled builds
will hapen at a specific CPU, or even ARM. I have tested locally and it
works as expected.

Fixes PR12794.

llvm-svn: 243656