Use the default `vixl::aarch64::MacroAssembler` constructor where sensible.

Change-Id: If5d07577778a61f07f95f305a286e0215161334a
diff --git a/src/code-buffer-vixl.h b/src/code-buffer-vixl.h
index 0aeabf1..b9fc1ce 100644
--- a/src/code-buffer-vixl.h
+++ b/src/code-buffer-vixl.h
@@ -36,7 +36,9 @@
 
 class CodeBuffer {
  public:
-  explicit CodeBuffer(size_t capacity = 4 * KBytes);
+  static const size_t kDefaultCapacity = 4 * KBytes;
+
+  explicit CodeBuffer(size_t capacity = kDefaultCapacity);
   CodeBuffer(void* buffer, size_t capacity);
   ~CodeBuffer();