cmake: include config-specific and LTO flags into TARANTOOL_C_FLAGS

Prior to this patch only the flags given by the CMAKE_C_FLAGS were
shown in the tarantool -v and Lua's tarantool.build.flags. Make
it also show the flags from CMAKE_C_FLAGS_<CONFIG> and CFLAGS_LTO
variables. The first one is config-specific set of flags (different
for Debug, Release, RelWithDebInfo and MinSizeRel configs), the
former is set in the cmake/lto.cmake if LTO is enabled.

Closes #8022

NO_DOC=build