Decouple CpuFeatures from serializer state.

Decouple CpuFeatures from serializer state.

Traditionally, we cross compile a snapshot iff the serializer is enabled.
This will change in the future.

Changes:
 - CpuFeatures probing is done once per process, depending on whether we
   cross compile.
 - CpuFeatures are consolidated into the platform-independent assembler.h
   as much as possible.
 - FLAG_enable_<feature> will only be checked at probing time (already the
   case for ARM).
 - The serializer state is cached by the MacroAssembler.
 - PlatformFeatureScope is no longer necessary.
 - CPUFeature enum values no longer map to CPUID bit fields.

R=svenpanne@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=21347

[Sven Panne, 2014-05-16 10:41:50]

lgtm

[Yang, 2014-05-16 15:19:06]

Committed patchset #1 manually as r21347 (presubmit successful).