New code layout - breaking change in cpu_features_macros.h (#194)

This commit helps with platform code separation (fixes #3). It should also help with the build as we can simply include all `impl_*.c` files regardless of OS / arch.

Note: this patch contains breaking changes in `include/cpu_features_macros.h`
 - `CPU_FEATURES_OS_LINUX_OR_ANDROID` does not exist anymore
 - `CPU_FEATURES_OS_FREEBSD`, `CPU_FEATURES_OS_ANDROID` and `CPU_FEATURES_OS_LINUX` are now mutually exclusive (i.e. `CPU_FEATURES_OS_ANDROID` does not imply `CPU_FEATURES_OS_LINUX`)
 - `CPU_FEATURES_OS_DARWIN` has been renamed into `CPU_FEATURES_OS_MACOS` to be able to target non-Mac Apple products (IOS, TV, WATCH). They are now targetable with `CPU_FEATURES_OS_IPHONE`. This matches Apple naming convention described in [this stackoverflow](https://stackoverflow.com/a/49560690).

src/define_introspection_and_hwcaps.inl

@@ -0,0 +1,26 @@
+// Copyright 2017 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "define_introspection.inl"
+#include "internal/hwcaps.h"
+
+#define LINE(ENUM, NAME, CPUINFO_FLAG, HWCAP, HWCAP2) \
+  [ENUM] = (HardwareCapabilities){HWCAP, HWCAP2},
+static const HardwareCapabilities kHardwareCapabilities[] = {
+    INTROSPECTION_TABLE};
+#undef LINE
+
+#define LINE(ENUM, NAME, CPUINFO_FLAG, HWCAP, HWCAP2) [ENUM] = CPUINFO_FLAG,
+static const char* kCpuInfoFlags[] = {INTROSPECTION_TABLE};
+#undef LINE