COMPILE_WARNING_AS_ERROR: Fix internal formatting of options table

In commit 76a08cd253 (COMPILE_WARNING_AS_ERROR: Add options to treat
warnings as errors, 2022-04-21, v3.24.0-rc1~173^2) we formatted the
options table entries as command-line string fragments.  Since they are
part of the `CMAKE_${lang}_COMPILE_OPTIONS_*` tables, they should be
formatted as `;`-separated lists of compiler options.