Go 1's math/rand would more accurately be called an additive lagged Fibonacci generator. The first publication of it is due to Green, Smith, and Klem [1].
Rob Pike and I exchanged mail with Don Mitchell (who wrote the original C version of the Go 1 generator) a few months back to see how he would describe the algorithm, and he said "As I recall Jim and I implemented Marsaglia's LFSR-like generator."
I think both descriptions (lagged Fibonacci and LFSR-like) are accurate in different ways, so either would be fine, but for the post I decided to use the original author's description.