sections_elf_shared: Do not access TLS of dead thread in finiTLSRanges()

finiTLSRanges() is called from the destructor of core.Thread. At this point,
the OS thread has already ceased to exist, so what was formerly a pointer
to _loadedDSOs is no longer valid.

In other words, `tdsos.reset()` was a use-after-free bug. It is unclear why
the issue didn't surface on Linux/FreeBSD yet; for example, glibc might not
actually re-use the TLS address range after a thread exits. On OS X, however,
this did quite frequently trigger a crash when running the Phobos unit tests,
since `tdsos` would have already been overwritten with unrelated contents.

src/rt/sections_elf_shared.d

@@ -149,7 +149,10 @@ version (Shared)
 
     void finiTLSRanges(Array!(ThreadDSO)* tdsos)
     {
-        tdsos.reset();
+        // Nothing to do here. tdsos used to point to the _loadedDSOs instance
+        // in the dying thread's TLS segment and as such is not valid anymore.
+        // The memory for the array contents was already reclaimed in
+        // cleanupLoadedLibraries().
     }
 
     void scanTLSRanges(Array!(ThreadDSO)* tdsos, scope ScanDG dg) nothrow
@@ -219,6 +222,8 @@ version (Shared)
             for (; tdso._addCnt > 0; --tdso._addCnt)
                 .dlclose(handle);
         }
+
+        // Free the memory for the array contents.
         _loadedDSOs.reset();
     }
 }