aarch64: Add cpu feature detection to libgcc

aarch64: Add cpu feature detection to libgcc

This is added to enable function multiversioning, but can also be used
directly.  The interface is chosen to match that used in LLVM's
compiler-rt, to facilitate cross-compiler compatibility.

The content of the patch is derived almost entirely from Pavel's prior
contributions to compiler-rt/lib/builtins/cpu_model.c. I have made minor
changes to align more closely with GCC coding style, and to exclude any code
from other LLVM contributors, and am adding this to GCC with Pavel's approval.

libgcc/ChangeLog:

	* config/aarch64/t-aarch64: Include cpuinfo.c
	* config/aarch64/cpuinfo.c: New file
	(__init_cpu_features_constructor) New.
	(__init_cpu_features_resolver) New.
	(__init_cpu_features) New.

Co-authored-by: Pavel Iliin <Pavel.Iliin@arm.com>