Update makefile dependencies on V8.

Depend on V8 using the architecture-specific LOCAL_STATIC_LIBRARIES
variables instead of testing TARGET_ARCH. This will allow libpac to be
removed from the 64-bit blacklist.

arm64 is an unsupported target arch for now, because the WebView copy of
V8 is not yet being built for arm64. This can be removed once WebView's
V8 is building.

Bug: 12452053
Change-Id: Ib78f5213e618a3914a5d85777dfdf82bb791229a