Document where architecture/abi-selection compile flags are allowed

Flags like -m32 can go in CMAKE_C_FLAGS when it is populated before the C language is enabled, typically through CFLAGS=-m32. They affect the initial detection of the compiler's target architecture and other information CMake uses to generate the build system.

Such flags should not be added later by add_compile_options or similar.

A section documenting this should be added to the cmake-buildsystem(7) manual, or another suitable location. The documentation of relevant variables and commands should be updated to reference it.