Portable Native Client port

- Add new configuration "pnacl" for targeting Portable Native Client (PNaCl) - Google's technology for safely running native code in the browser. NACL_SDK_ROOT environment variable must be properly setup for the build, and PATH must include the directory with toolchain binaries ($NACL_SDK_ROOT/<OS>_pnacl/bin).
- Provides tuned kernel sizes and kernels optimized for Portable SIMD (based on Clang's vector extensions). Portable SIMD extensions are available in Chromium 36+, when targeting earlier versions the build will use different kernel sizes and scalar code.
- The test suite makefile is modifier to incorporate additional steps required to run PNaCl applications: linked code must be finalized (i.e. converted to a stable bitcode format, linked code is just LLVM bitcode, which is neither forward nor backward compatible with different Chromium and SDK versions) and translated to Native Client executable (unlike PNaCl, these are architecture-dependent, and can be executed with Secure ELF Loader). The makefile also got new targets run-amd64, run-x86, and run-arm to run a test binary under appropriate Secure ELF Loader.