Do it all at once in preparation for Go 1.18 support.
To make this commit, I've simply modified the `fmt-check` Makefile
target to rewrite files instead of listing the differences. So this is a
fully mechanical change, it should not have introduced any errors.
This is mainly useful to be able to run `tinygo test`, for example:
tinygo test -target=cortex-m-qemu -v math
This is not currently supported, but will be in the future.