Sorry - I'm confused. 4.2.8 is on kernel.org, but to bump to 4.2.9 I pick up patches from ... ?
It was released by Canonical, they have 4.2 as a long-term kernel in an Ubuntu version. Official name is 4.2.8-ckt1. Then we have some of our own patches.
Running 5.2.0 "gcc -v ..." shows no "--with-arch=i486 --with-tune=i686" which was present in 4.9.1:
It should work with just a 486 target though, which is correct on both.
edit: From your earlier "gcc -v" output, we see that gcc applied -march=i486. This is what the --with-arch option would do, it sets the default -march, it doesn't affect the new gcc's build process.
What drawbacks would be possible from using gcc4.9.1-compiled kernel modules with 4.x kernel?
It won't work if you mix compilers. The kernel and modules must be compiled by the same gcc version. If you build it all, the gcc version doesn't matter much, as the kernel supports every gcc version from 3.4 up, IIRC.
Applied your patch, thanks.