runtime: Fix setting of cpu features for amd64

Author: nimelehin

src/runtime/proc.go

@@ -623,7 +623,7 @@ func cpuinit() {
 	// Support cpu feature variables are used in code generated by the compiler
 	// to guard execution of instructions that can not be assumed to be always supported.
 	switch GOARCH {
-	case "386", "AMD64":
+	case "386", "amd64":
 		x86HasPOPCNT = cpu.X86.HasPOPCNT
 		x86HasSSE41 = cpu.X86.HasSSE41
 		x86HasFMA = cpu.X86.HasFMA