Remove typedefs.h from webrtc/ root (part 1)

Split out webrtc-specific #defines from typedefs.h, into rtc_base/annotations.h and rtc_base/arch.h.
Also removes the curiously named WEBRTC_CPU_DETECTION #define.

BUG=webrtc:6854

Review-Url: https://codereview.webrtc.org/3007253002
Cr-Commit-Position: refs/heads/master@{#19752}
diff --git a/webrtc/modules/audio_coding/neteq/neteq_impl.cc b/webrtc/modules/audio_coding/neteq/neteq_impl.cc
index 7858e84..f4ace1b 100644
--- a/webrtc/modules/audio_coding/neteq/neteq_impl.cc
+++ b/webrtc/modules/audio_coding/neteq/neteq_impl.cc
@@ -935,7 +935,7 @@
           static_cast<uint32_t>(output_size_samples_));
       // Skipping break on purpose. Execution should move on into the
       // next case.
-      FALLTHROUGH();
+      RTC_FALLTHROUGH();
     }
     case kAudioRepetition: {
       // TODO(hlundin): Write test for this.