Historically, it hasn't really taken "serious competition" to invite threats. There's a reason that the best-known open-source ARM implementation explicitly only targets ARMv2 (which dates from the days when the "A" stood for "Acorn") [1].

That said, people who just want to play with ARM-based hardware can get pretty far by leaning on the GNU support and third-party documentation (there are some Game Boy Advance documents that cover the ARMv4 instruction set, for example).

[1]: http://opencores.org/project,amber