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}