The register r0 was used unconditionally. This is a bug: the compiler
doesn't know it is clobbered and might consider it alive across the
inline assembly expression.
The fix is simple. It could probably be optimized, but this should at
least fix the bug.
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.
