[libc] Stub TLS functions on the GPU temporarily (#108267)

Summary:
There's an extern weak symbol for this, we should just factor these into
a more common interface. Stub them temporarily to make the bots happy.
PTXAS does not handle extern weak.

libc/startup/gpu/amdgpu/start.cpp

@@ -17,6 +17,9 @@ extern "C" int main(int argc, char **argv, char **envp);
 
 namespace LIBC_NAMESPACE_DECL {
 
+// FIXME: Factor this out into common logic so we don't need to stub it here.
+void teardown_main_tls() {}
+
 DataEnvironment app;
 
 extern "C" uintptr_t __init_array_start[];

libc/startup/gpu/nvptx/start.cpp

@@ -19,6 +19,9 @@ namespace LIBC_NAMESPACE_DECL {
 
 DataEnvironment app;
 
+// FIXME: Factor this out into common logic so we don't need to stub it here.
+void teardown_main_tls() {}
+
 extern "C" {
 // Nvidia's 'nvlink' linker does not provide these symbols. We instead need
 // to manually create them and update the globals in the loader implememtation.