tracing/runtime: Fix build for Go1.12

This continues b46436bd (tracing/runtime: Try to add support for Go1.12):

Move `+build ...` constraint in assembly files to the top, as suggested
by Russ Cox:

	https://github.com/golang/go/issues/31410#issuecomment-482599134

Then go stops complaining about it.

The build constraint had no effect previously, but it was not caught
because getg is used only on race builds and I was testing on amd64
only, which is practically almost the only ISA supported by race
detector as of Go1.11 .

tracing/internal/xruntime/runtime_g_386.s

-#include "textflag.h"
-
 // +build 386
 
+#include "textflag.h"
+
 // func getg() *g
 TEXT ·getg(SB),NOSPLIT,$0-8
 	MOVL (TLS), AX

tracing/internal/xruntime/runtime_g_amd64.s

-#include "textflag.h"
-
 // +build amd64 amd64p
 
+#include "textflag.h"
+
 // func getg() *g
 TEXT ·getg(SB),NOSPLIT,$0-8
 	MOVQ (TLS), R14