libunwindstack: Add Memory::ReadTag() function for reading memory tags.

This uses an experimental Linux kernel API for reading the tags across
processes using ptrace.

Bug: 135772972

Test: Unit tests pass.
Change-Id: Ib1a09d9219166011de80cf250b756bb8a4bcdb0a
diff --git a/libunwindstack/MemoryRemote.h b/libunwindstack/MemoryRemote.h
index db367d6..dd09c88 100644
--- a/libunwindstack/MemoryRemote.h
+++ b/libunwindstack/MemoryRemote.h
@@ -32,6 +32,7 @@
   virtual ~MemoryRemote() = default;
 
   size_t Read(uint64_t addr, void* dst, size_t size) override;
+  long ReadTag(uint64_t addr) override;
 
   pid_t pid() { return pid_; }