Clean up public headers using CMake `configure_file` (@ONLY) for system header subsitution

[bin-wang]

#8025 Introduces inline conditional include logic for system headers in exported public API headers. For example, in dr_defines.h (generated from globals_api.h):

#ifdef LINUX_KERNEL
#    include <linux/stdarg.h>
#else
#    include <stdarg.h> /* for varargs */
#endif

This duplicates the logic in stdarg_wrapper.h. However, because stdarg_wrapper.h is not exported, we cannot use it in dr_defines.h. This clutters exported public headers and duplicates the wrapper logic.

We think the proper solution is to use CMake configure-time variable substitution (configure_file with @ONLY) when exporting public API headers, such that system header dependencies are resolved at build configuration time.