xtensa: use 8-byte alignment

This is probably not necessary on Espressif chips, but let's strictly
follow the ABI to be sure.

src/runtime/arch_xtensa.go

@@ -9,9 +9,9 @@ const TargetBits = 32
 
 const deferExtraRegs = 0
 
-// Align on a word boundary.
+// The largest alignment according to the Xtensa ABI is 8 (long long, double).
 func align(ptr uintptr) uintptr {
-	return (ptr + 3) &^ 3
+	return (ptr + 7) &^ 7
 }
 
 func getCurrentStackPointer() uintptr {