ART: Add support for "silvermont" variant

The variant is already supported in the build system and wired up
for the Nexus Player (fugu). The values are copied from the build
system and correspond to documentation in, e.g.,
http://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-optimization-manual.pdf
chapter 15, section 1.1.

Change-Id: I9facaa9d5b8bae3d98cf48f2969d02d11ebfe0e4