ARM: Fix build flags for snapshot generation.

The existing code ignores armv7 and arm_fpu flags since r14288.

R=jkummerow@chromium.org, rodolph.perfetta@gmail.com

Review URL: https://chromiumcodereview.appspot.com/16313003

Patch from Jakob Kummerow <jkummerow@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14998 ce2b1a6d-e550-0410-aec6-3dcde31c8c00