[color=#66cc33]The following tutorial is not officially supported, and could potentially harm your system, proceed at your own risk.[/color]
[hr]
now … 
git clone (or unZip) this https://github.com/manjaro/packages-core :idea:
and in 417 subdir run
makepkg
as usual 
follow guide: https://wiki.parabola.nu/Kernel_native_compilation
more or less 
you get a working kernel and initrd, but there may well be graphics driver problems,
so you’re stuck in the bash shell. :-/
with https://en.wikipedia.org/wiki/Dracut_(software) you get a working RAMDISK for NRR / Manja real quick. 
else use your regular mkinitcpio with -P possibly
One more time, you need to start adding a disclaimer that these tutorials of yours are NOT officially supported.
-march=native no longer works right away. linker breaks now. may still be poss somehow.
always keep a backup copy of a known working “.config” , the corresponding kernel binary “bzImage” and a working initrd in a safe subdir to fall back on in case of crash / hang. In .config (i.e. make xconfig) you have to tread carefully since many options will render the kernel unbootable without further meddling.
keep in mind: you get boot fail if:
mismatch of either one component: kernel-binary “bzImage”, initrd , and stuff under /lib/modules
e.g. at some stage /lib/modules/4.17.0-1-MANJARO+
will be filled by “time sudo make modules_install” mind the + at the end, without it: BOOT FAIL 