cmake: add support for --toolchain command argument

Help/manual/OPTIONS_BUILD.txt

Help/manual/cmake-toolchains.7.rst

Help/release/dev/cmake-toolchain-command.rst

Source/cmake.cxx

Source/cmake.h

Tests/RunCMake/CommandLine/RunCMakeTest.cmake

Tests/RunCMake/CommandLine/Toolchain/CMakeLists.txt

Tests/RunCMake/CommandLine/Toolchain/toolchain.cmake

Tests/RunCMake/CommandLine/toolchain-no-arg-result.txt

Tests/RunCMake/CommandLine/toolchain-no-arg-stderr.txt

.../RunCMake/CommandLine/toolchain-valid-abs-path-result.txt

.../RunCMake/CommandLine/toolchain-valid-abs-path-stderr.txt

...ake/CommandLine/toolchain-valid-rel-build-path-result.txt

...ake/CommandLine/toolchain-valid-rel-build-path-stderr.txt

...CMake/CommandLine/toolchain-valid-rel-src-path-result.txt

...CMake/CommandLine/toolchain-valid-rel-src-path-stderr.txt