remove the concept of "sub-architecture"

in favor of CPU features. Also rearrange the `std.Target`
data structure.

 * note: `@import("builtin")` was already deprecated in favor of
   `@import("std").builtin`.
 * `std.builtin.arch` is now deprecated in favor of
   `std.builtin.cpu.arch`.
 * `std.Target.CpuFeatures.Cpu` is now `std.Target.Cpu.Model`.
 * `std.Target.CpuFeatures` is now `std.Target.Cpu`.
 * `std.Target` no longer has an `arch` field. Instead it has a
   `cpu` field, which has `arch`, `model`, and `features`.
 * `std.Target` no longer has a `cpu_features` field.
 * `std.Target.Arch` is moved to `std.Target.Cpu.Arch` and
   it is an enum instead of a tagged union.
 * `std.Target.parseOs` is moved to `std.Target.Os.parse`.
 * `std.Target.parseAbi` is moved to `std.Target.Abi.parse`.
 * `std.Target.parseArchSub` is only for arch now and moved
    to `std.Target.Cpu.Arch.parse`.
 * `std.Target.parse` is improved to accept CPU name and features.
 * `std.Target.Arch.getBaselineCpuFeatures` is moved to
   `std.Target.Cpu.baseline`.
 * `std.Target.allCpus` is renamed to `std.Target.allCpuModels`.
 * `std.Target.defaultAbi` is moved to `std.Target.Abi.default`.
 * Significant cleanup of aarch64 and arm CPU features, resulting in
   the needed bit count for cpu feature set going from 174 to 138.
 * Add `std.Target.Cpu.Feature.Set.addFeatureSet` for merging
   feature sets together.

`-target-feature` and `-target-cpu` are removed in favor of
`-mcpu`, to conform to established conventions, and it gains
additional power to support cpu features. The syntax is:
-mcpu=name+on1+on2-off1-off2

closes #4261

src/error.cpp

@@ -59,7 +59,6 @@ const char *err_str(Error err) {
         case ErrorIsAsync: return "is async";
         case ErrorImportOutsidePkgPath: return "import of file outside package path";
         case ErrorUnknownCpu: return "unknown CPU";
-        case ErrorUnknownSubArchitecture: return "unknown sub-architecture";
         case ErrorUnknownCpuFeature: return "unknown CPU feature";
         case ErrorInvalidCpuFeatures: return "invalid CPU features";
         case ErrorInvalidLlvmCpuFeaturesFormat: return "invalid LLVM CPU features format";