Just to make sure, you always restart the X server between building a Mesa
commit and testing xterm against it, right? And you're making sure that the X
server picks up your self-built radeonsi_dri.so for glamor?

If so, since that commit doesn't seem to affect the xterm issue, let's assume
for now that it's separate. Please file a new report about the GTK issue, also
against radeonsi, with the information from git bisect.