Add Xtensa Port

[zephyrbot]

Reported by Anas Nashif:

Support the Xtensa architecture with a new a port

• xtensa: simplify linker scripts #3199: xtensa: simplify linker scripts
• xtensa: Zephyr SDK build and emulation support #3184: xtensa: Zephyr SDK build and emulation support
• xtensa: enable build target for real hardware #3183: xtensa: enable build target for real hardware
• scalable solution for test case stack sizes #3181: scalable solution for test case stack sizes
• xtensa: tinycrypt does not build #3175: xtensa: tinycrypt does not build
• xtensa: correct all checkpatch issues #3166: xtensa: correct all checkpatch issues
• xtensa: switch to clang-based frontend #3165: xtensa: switch to clang-based frontend
• xtensa build defines Kconfigs with lowercase names #3164: xtensa build defines Kconfigs with lowercase names
• xtensa: Support C++ #3156: xtensa: fix C++ support
• xtensa: fix tests/kernel/mem_protect/stackprot #3155: xtensa: fix test_stackprot
• xtensa: implement XIP support #3130: xtensa: implement XIP support
• extend tests/kernel/arm_irq_vector_table to other platforms #3098: extend tests/kernel/arm_irq_vector_table to other platforms
• Xtensa integration #3018: Xtensa integration
• Bug in Zephyr OS high-precision timings sub-system (function sys_cycle_get_32()) #3012: Bug in Zephyr OS high-precision timings sub-system (function sys_cycle_get_32())

(Imported from Jira ZEP-1468)