v8: Hardfloat does not imply VFPv3, only VFPv2.

Raspberry Pi is an example.

BUG=v8:2393

Review URL: https://chromiumcodereview.appspot.com/11570061
Patch from Chi-Thanh Christopher Nguyen <nguyenchithanh@gmail.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13232 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

This is a backport of v8/v8@44419ad089b729a1aab4a5f27c0833e2ce69d28b.
configure: define "arm_fpu" and "arm_neon" for ARM

v8's common.gypi file expects them to be defined now.

Closes #4534.