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/video_coding/jitter_buffer.cc b/webrtc/modules/video_coding/jitter_buffer.cc
index a556080..eaf09be 100644
--- a/webrtc/modules/video_coding/jitter_buffer.cc
+++ b/webrtc/modules/video_coding/jitter_buffer.cc
@@ -749,7 +749,7 @@
           frame_event_->Set();
         }
       }
-      FALLTHROUGH();
+      RTC_FALLTHROUGH();
     }
     // Note: There is no break here - continuing to kDecodableSession.
     case kDecodableSession: {