Build v8 snapshot with correct default float configuration on Linux ARM

V8 currently defaults to arm_float_abi="hard" and arm_use_neon=true but
the V8 snapshot defaults to arm_float_abi="softfp" and arm_use_neon=false
on Linux ARM builds.

This patch makes both targets default to hard + neon by changing the
"is simulator build" check from comparing current_cpu with v8_current_cpu
to comparing target_cpu with v8_target_cpu instead. Similarly to how it is
checked in v8/BUILD.gn.

BUG=662856

Review-Url: https://codereview.chromium.org/2483153004
Cr-Commit-Position: refs/heads/master@{#430776}