ARM: Changed the handling of compiletime CPU feature detection

ARM: Changed the handling of compiletime CPU feature detection

Always use CpuFeaturesImpliedByCompiler() when selecting CPU features. This checks both for CAN_USE_ARMV7_INSTRUCTIONS and CAN_USE_VFP_INSTRUCTIONS and for GCC preprocessor symbols. This will support using the CAN_USE_XXX for a simulator build used for generating a snapshot followed by a crosscompile using -march= and -mfpu= for selecting the (minimal) target device CPU features. The snapshot will use instructions based on the CAN_USE_XXX whereas the target will at least use features based on both CAN_USE_XXX and -march= and -mfpu=, but will try runtime CPU feature detection a well looking for somethis better.

Remove the compiler based CPU feature detection from the OS::CpuFeaturesImpliedByPlatform() as it did not belong there. Also was already in the CpuFeaturesImpliedByCompiler().

Add the variable 'v8_can_use_vfp_instructions' to the GYP file which can be used to turn on CAN_USE_VFP_INSTRUCTIONS when building V8. I did not add any -mfpu= cflags for this, as there are several options here (e.g. vfp and neon).

R=erik.corry@gmail.com, karlklose@chromium.org

BUG=none
TEST=none

Committed: http://code.google.com/p/v8/source/detail?r=7754

[Karl Klose, 2011-05-02 12:36:32]

LGTM.

[Erik Corry, 2011-05-03 06:49:25]

lgtm