[ARM,CDE] Implement CDE feature test macros

Summary:
This patch implements feature test macros for the CDE extension
according to the upcoming ACLE specification.

The following 2 macros are being added:
- __ARM_FEATURE_CDE - defined as '1' when any coprocessor is
  configured as a CDE coprocessor
- __ARM_FEATURE_CDE_COPROC - defined as an 8-bit mask, each bit of the
  mask corresponds to a coprocessor and is set when the corresponding
  coprocessor is configured as CDE (and cleared otherwise).

The patch also exposes the value of __ARM_FEATURE_CDE_COPROC in the
target-independent method TargetInfo::getARMCDECorpocMask, the method
will be used in follow-up patches implementing semantic checks of CDE
intrinsics (we want to diagnose the cases when CDE intrinsics are used
with coprocessors that are not configured as CDE).

Reviewers: simon_tatham, dmgreen, ostannard, MarkMurrayARM

Reviewed By: simon_tatham

Subscribers: kristof.beyls, danielkiss, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75843