[NFC][AArch64] Split out backend features

[NFC][AArch64] Split out backend features

This patch splits backend features currently
hidden behind architecture versions.

For example, currently the only way to activate
complex numbers extension is targeting an v8.3
architecture, where after the patch this extension
can be added separately.

This refactoring is required by the new command lines proposal:
http://lists.llvm.org/pipermail/llvm-dev/2018-September/126346.html

Reviewers: DavidSpickett, olista01, t.p.northover

Subscribers: kristof.beyls, bryanpkc, javed.absar, pbarrio

Differential revision: https://reviews.llvm.org/D54633

--

It was reverted in rL348249 due a	build bot failure in one of the
regression tests:
http://lab.llvm.org:8011/builders/llvm-clang-x86_64-expensive-checks-win/builds/14386

The problem seems to be that FileCheck behaves
different in windows and linux. This new patch
splits the test file in multiple,
and does more exact pattern matching attempting
to circumvent the issue.

llvm-svn: 348493