Add HL_HEXAGON_SIM_REMOTE env var for path to HVX Simulator
Add HL_HEXAGON_SIM_REMOTE env var for path to HVX Simulator

Also rename other env vars to use HL_HEXAGON_ instead of HL_HEX_ prefix.


Former-commit-id: d52be8f4b35e647e29093c6d30d32dacf73ed44e
darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide