ART: Convert pointer size to enum

Move away from size_t to dedicated enum (class).

Bug: 30373134
Bug: 30419309
Test: m test-art-host
Change-Id: Id453c330f1065012e7d4f9fc24ac477cc9bb9269
diff --git a/imgdiag/imgdiag.cc b/imgdiag/imgdiag.cc
index f5669d7..21a0ca0 100644
--- a/imgdiag/imgdiag.cc
+++ b/imgdiag/imgdiag.cc
@@ -285,7 +285,7 @@
                         const backtrace_map_t& boot_map)
     SHARED_REQUIRES(Locks::mutator_lock_) {
     std::ostream& os = *os_;
-    const size_t pointer_size = InstructionSetPointerSize(
+    const PointerSize pointer_size = InstructionSetPointerSize(
         Runtime::Current()->GetInstructionSet());
 
     std::string file_name =