Implement Math.log2 via ported extract from fdlibm.

Adapted from Raymond Toy's (rtoy@chromium.org) port, extracted from fdlibm's pow implementation.

R=rtoy@chromium.org
BUG=v8:3579
LOG=N

Review URL: https://codereview.chromium.org/786823003

Cr-Commit-Position: refs/heads/master@{#25768}
Disable invalid DCHECK_EQ in serializer.cc

BUG=none
R=mvstanton@chromium.org,yangguo@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/791363002

Cr-Commit-Position: refs/heads/master@{#25769}
remove Isolate::debugger_initialized_

It's never used.

BUG=none
R=dcarney@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/797543002

Cr-Commit-Position: refs/heads/master@{#25770}