- Add Silvermont (--cpu=slm) option for llvm 3.4+.

- Change default Sandybridge isa name to avx1-i32x8 from avx-i32x8,
  to conform with replacement of avx-i32x8 by avx1-i32x8 everywhere else.
- Add "target-cpu" attribute, when using AttrBuilder, to correct a problem
  whereby llvm would switch from the command line cpu setting
  to the native (auto-detected) cpu setting on second and subsequent
  functions. e.g. if I wanted to build for Silvermont on a Sandy Bridge
  machine, ispc/llvm would correctly use Silvermont and turn on the
  Silvermont scheduler. For the second and subsequent functions,
  it would auto-detect Sandy Bridge, but still run the Silvermont
  scheduler.