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-Original-Commit-Position: refs/heads/master@{#19752}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: a895836321e68b67ca2081c3bdbf6c08d8ee4b57