Generate g for Go 1.12; no changes here compared to Go 1.11, however
go1.12 build build fails because of +build in .s file:

	# lab.nexedi.com/kirr/go123/tracing/internal/xruntime
	./runtime_g_amd64.s:3:1: +build comment must appear before package clause and be followed by a blank line

Build problem reported upstream:

	https://github.com/golang/go/issues/31410