Hmmm... I think static linking against libc would be a very bad idea - as literally every other library depends on this, it would cause a lot of mess and incompatibility.
I think the problem is elsewhere, as my kindle also has libc 2.20 (and runs Gargoyle just fine).
I imagine that you have a cross-compile toolchain already installed. You might try to build my repo:
https://github.com/kbarni/garglk/ (and replace the files in the bin/ folder of the KUAL package from your build)