You must have Go v1.14+ already installed on your machine in order to install TinyGo
brew tap tinygo-org/tools
brew install tinygo
$ tinygo version
tinygo version 0.15.0 darwin/amd64 (using go version go1.15 and LLVM version 10.0.1)
brew tap osx-cross/avr
brew install avr-gcc
brew install avrdude