Native trigonometry

Added
1. Support for ISA with native trigonometry
2. Double precision tests for transendentals

Problems:
1. knc-i1x16.h works, but knc.h crashes ICC on

    4 / 40 tests FAILED compilation:
            ./tests/transcendentals-0-2.ispc
            ./tests/transcendentals-1-3.ispc
            ./tests/transcendentals-5-2.ispc
            ./tests/transcendentals-6-3.ispc

 Have no idea why.. The code for trigonometry in both is the same., and ispc generates identical C++ (modulo include file).