build: Stop using dolt
This does regress "make -12" from 0.7s to 0.9s on my
Coffee Lake machine, but even on slower hardware this
will not amount to a noticable slowdown.
On the other hand using dolt can create problems for
people doing cross-compilation, e.g. Yocto has two
hacks just for dolt in kmod:
https://git.yoctoproject.org/cgit/cgit.cgi/poky/tree/meta/recipes-kernel/kmod/kmod.inc?id=
a17abae00785c76cfffe5381a22fb2c86b982e82
(Lucas: remove leftover entry in Makefile and reformat commit message)