Move variable only used on aarch64.

The kTagGranuleSize is only used in aarch64, so you can get
unused variable warnings for other targets. Move this definition
closer to where it's used and in aarch64 code.

Test: Build with unused checking on.
Change-Id: I9baf660430508ee3ca1efb62585ee1b038c6b0f3
diff --git a/debuggerd/debuggerd_test.cpp b/debuggerd/debuggerd_test.cpp
index 08619b9..962097e 100644
--- a/debuggerd/debuggerd_test.cpp
+++ b/debuggerd/debuggerd_test.cpp
@@ -86,7 +86,6 @@
 #define ARCH_SUFFIX ""
 #endif
 
-constexpr size_t kTagGranuleSize = 16;
 constexpr char kWaitForDebuggerKey[] = "debug.debuggerd.wait_for_debugger";
 
 #define TIMEOUT(seconds, expr)                                     \
@@ -742,6 +741,8 @@
 }
 
 #if defined(__aarch64__)
+constexpr size_t kTagGranuleSize = 16;
+
 static uintptr_t CreateTagMapping() {
   // Some of the MTE tag dump tests assert that there is an inaccessible page to the left and right
   // of the PROT_MTE page, so map three pages and set the two guard pages to PROT_NONE.