[AArch64] Split out processor/tuning features

Following on from an earlier patch that introduced support for -mtune
for AArch64 backends, this patch splits out the tuning features
from the processor features. This gives us the ability to enable
architectural feature set A for a given processor with "-mcpu=A"
and define the set of tuning features B with "-mtune=B".

It's quite difficult to write a test that proves we select the
right features according to the tuning attribute because most
of these relate to scheduling. I have created a test here:

  CodeGen/AArch64/misched-fusion-addr-tune.ll

that demonstrates the different scheduling choices based upon
the tuning.

Differential Revision: https://reviews.llvm.org/D111551