ARMClang: Add support for Clang-based ARM compiler

Help/release/dev/armclang.rst

Help/variable/CMAKE_LANG_COMPILER_ID.rst

Modules/CMakeCompilerIdDetection.cmake

Modules/CMakeDetermineASMCompiler.cmake

Modules/CMakeDetermineCCompiler.cmake

Modules/CMakeDetermineCXXCompiler.cmake

Modules/CMakeDetermineCompilerId.cmake

Modules/Compiler/ARMCC-ASM.cmake

Modules/Compiler/ARMClang-ASM.cmake

Modules/Compiler/ARMClang-C-FeatureTests.cmake

Modules/Compiler/ARMClang-C.cmake

Modules/Compiler/ARMClang-CXX-FeatureTests.cmake

Modules/Compiler/ARMClang-CXX.cmake

Modules/Compiler/ARMClang-DetermineCompiler.cmake

Modules/Compiler/ARMClang.cmake