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

Review URL: http://codereview.chromium.org//6904164

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7754 ce2b1a6d-e550-0410-aec6-3dcde31c8c00