...The option EPREFIX of Gentoo prepends semi-automatically /data/data/com.spartacusrex.spartacuside/gentoo_armv6l to all paths, at compile time. Limited to ARM (similar to armel/raspian).
I am using gentooandroid at work, for everyday tasks (firefox, pdflatex, gnucash, sshfs, ...). The installation needs 1.8Go free space at least, on my phone it currently takes 6 Go.
Installation instructions: https://sf.net/p/gentooandroid/wiki