reflect/makefunc: move amd64-specific definitions

Move amd64-specific definitions from general makefunc.go to
amd64-specific makefunc_amd64.go.

For golang#38783

src/reflect/makefunc.go

@@ -202,16 +202,6 @@ var dispatchToMethodAddr = abi.FuncPCABIInternal(dispatchToMethod)
 // at runtime. Initially, it's the actual code address of dispatchToMethod.
 var currentMethodKey = dispatchToMethodAddr
 
-const (
-	// dispatchStep is the number of bytes we should call deeper into
-	// dispatchToMethod with each generated method.
-	dispatchStep = 5 // FIXME: AMD64 specific
-
-	// dispatchLimit is the upper limit for currentMethodKey. Once this limit
-	// is reached, we need to switch to a different page.
-	dispatchLimit = dispatchStep * 812 // FIXME: AMD64 specific
-)
-
 // methodSlotsExhausted is a drop-in for when there are no more method slots.
 // It will cause the method to panic when called. This late-failure behaviour
 // makes it possible to successfully call StructOf even if no more slots for

src/reflect/makefunc_amd64.go

@@ -11,6 +11,16 @@ import (
 	"unsafe"
 )
 
+const (
+	// dispatchStep is the number of bytes we should call deeper into
+	// dispatchToMethod with each generated method.
+	dispatchStep = 5
+
+	// dispatchLimit is the upper limit for currentMethodKey. Once this limit
+	// is reached, we need to switch to a different page.
+	dispatchLimit = dispatchStep * 812
+)
+
 // getMethodImpl returns the pointer to the closure for the specified key.
 // It is called from the ASM dispatch code.
 func getMethodImpl(