Honor CMAKE_*_LINKER_FLAGS[_<CONFIG>]_INIT set in toolchain files

Document these variables.

Change our convention for setting these variables from:

    set(CMAKE_EXE_LINKER_FLAGS_INIT "...")

to

    string(APPEND CMAKE_EXE_LINKER_FLAGS_INIT " ...")

so that any value previously set by a toolchain file will be used.