Enable -Wconversion for thread.cc

This should help prevent bugs due to unexpected implicit integer
conversions.

Some collateral changes were needed as well to limit the number of casts
that had to be introduced.

Bug: 165843530
Test: m test-art-host-gtests
Change-Id: I091122827001ab335c7e140864f67cdf90fcf8b4
diff --git a/runtime/stack_map.h b/runtime/stack_map.h
index 2065a79..0d289b8 100644
--- a/runtime/stack_map.h
+++ b/runtime/stack_map.h
@@ -413,7 +413,7 @@
     return stack_maps_.GetInvalidRow();
   }
 
-  StackMap GetStackMapForNativePcOffset(uint32_t pc, InstructionSet isa = kRuntimeISA) const;
+  StackMap GetStackMapForNativePcOffset(uintptr_t pc, InstructionSet isa = kRuntimeISA) const;
 
   // Dump this CodeInfo object on `vios`.
   // `code_offset` is the (absolute) native PC of the compiled method.