Trouble is there were several dozen possibilities, though C narrows the options down (there were various ARM assemblers, also often mutually incompatible to the point where a serious rewrite might have been needed).
Various people first used the leaked SDK from Nintendo to build things (agb toolchains some used to call it).
VisualHAM was a thing for a while
https://www.coranac.com/tonc/text/setup.htm#sec-env
Things that would eventually roll into devkitpro/devkitarm. You also have the problems that many things changed over the years and as of a few years back the people behind devkitpro have gone hammer and tongs* after every download of an old version for reasons I have never quite been able to determine (they say because source was lost, they also say because it might download malware in the future if someone takes over the URL). This would further complicate the compiling of legacy code, assuming it ever would have.
*as in filing DMCA requests, contacting hosts and causing a fair bit of trouble for people.