Help: Document macro argument caveats in more detail

Add notes about macro arguments in the foreach, if, and list commands.
Add a section to the macro command documentation explaining in detail
how macro arguments are not variables.
CMakeDetermineCompilerId: Look for internal file only on host

The find_file this module uses to locate the compiler id source file
must always look on the host and never in CMAKE_FIND_ROOT_PATH, even
when a toolchain file has

 set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)

Add NO_CMAKE_FIND_ROOT_PATH to the find_file call to avoid rerooting.
Add compiler target compile options.

For clang, this allows passing -target <triple> to the compiler, and
for qcc, -V<arch> using toolchain files containing something like

 set(triple arm-linux-gnueabihf)
 set(CMAKE_C_COMPILER "/usr/bin/clang")
 set(CMAKE_C_COMPILER_TARGET ${triple})
 set(CMAKE_CXX_COMPILER "/usr/bin/clang++")
 set(CMAKE_CXX_COMPILER_TARGET ${triple})

or

 set(arch gcc_ntoarmv7le)
 set(CMAKE_C_COMPILER /opt/qnx650/host/linux/x86/usr/bin/qcc)
 set(CMAKE_C_COMPILER_TARGET ${arch})
 set(CMAKE_CXX_COMPILER /opt/qnx650/host/linux/x86/usr/bin/QCC)
 set(CMAKE_CXX_COMPILER_TARGET ${arch})

Both clang and qcc are inherently cross compiler( driver)s.

When cross-compiling with clang, use the CMAKE_${lang}_COMPILER_TARGET
as the _CMAKE_TOOLCHAIN_PREFIX to find the appropriate binutils.

When cross-compiling with QNX qcc, use the CMAKE_${lang}_COMPILER_TARGET
to set the appropriate _CMAKE_TOOLCHAIN_PREFIX.
Allow toolchain files to specify an external toolchain.

Clang can compile code, but uses the gcc tools for other tasks such
as linking. The -gcc-toolchain option can be used for that, but
generalize so that other compilers can be treated the same.

If such a location is specified, use it as a hint for finding
the binutils executables.
Add CMAKE_SYSROOT variable to set --sysroot when cross compiling.

As CMAKE_ROOT_FIND_PATH can be a list, a new CMAKE_SYSROOT is
introduced, which is never a list.

The contents of this variable is passed to supporting compilers
as --sysroot. It is also accounted for when processing implicit
link directories reported by the compiler, and when generating
RPATH information.
Merge topic 'cross-compiling-toolchain-variables'

7cd65c9 Add CMAKE_SYSROOT variable to set --sysroot when cross compiling.
5096967 Allow toolchain files to specify an external toolchain.
76552d5 Add compiler target compile options.
f41ecd1 CMakeDetermineCompilerId: Look for internal file only on host
Merge topic 'macro-args-docs'

f0db2e3 Help: Document macro argument caveats in more detail