Cadence Tensilica Xtensa C/C++ Compiler (XCC)

Obtain Tensilica Software Development Toolkit targeting the specific SoC on hand. This usually contains two parts:
- The Xtensa Xplorer which contains the necessary executables and libraries.
- A SoC-specific add-on to be installed on top of Xtensa Xplorer.
  - This add-on allows the compiler to generate code for the SoC on hand.
Install Xtensa Xplorer and then the SoC add-on.
- Follow the instruction from Cadence on how to install the SDK.
- Depending on the SDK, there are two set of compilers:
  - GCC-based compiler: xt-xcc and its friends.
  - Clang-based compiler: xt-clang and its friends.
Make sure you have obtained a license to use the SDK, or has access to a remote licensing server.
Set these environment variables:
- Set ZEPHYR_TOOLCHAIN_VARIANT to xcc or xt-clang.
- Set XTENSA_TOOLCHAIN_PATH to the toolchain installation directory.
- Set XTENSA_CORE to the SoC ID where application is being targeting.
- Set TOOLCHAIN_VER to the Xtensa SDK version.

For example, assuming the SDK is installed in /opt/xtensa, and using the SDK for application development on intel_adsp_cavs15, setup the environment using:

# Linux
export ZEPHYR_TOOLCHAIN_VARIANT=xcc
export XTENSA_TOOLCHAIN_PATH=/opt/xtensa/XtDevTools/install/tools/
export XTENSA_CORE=X6H3SUE_RI_2018_0
export TOOLCHAIN_VER=RI-2018.0-linux

To use Clang-based compiler:
- Set ZEPHYR_TOOLCHAIN_VARIANT to xt-clang.
- Note that the Clang-based compiler may contain an old LLVM bug which results in the following error:
```
/tmp/file.s: Assembler messages:
/tmp/file.s:20: Error: file number 1 already allocated
clang-3.9: error: Xtensa-as command failed with exit code 1
```
  If this happens, set XCC_NO_G_FLAG to 1.
  - For example:
    # Linux export XCC_NO_G_FLAG=1
- Also note that setting XCC_USE_CLANG to 1 and ZEPHYR_TOOLCHAIN_VARIANT to xcc is deprecated. Set ZEPHYR_TOOLCHAIN_VARIANT to xt-clang instead.