std: Work around some failing 'run' tests when valgrinding. #7224

Under valgrind on 64->32 cross compiles the dynamic linker is emitting
some error messages on stderr, which interferes with the tests that
are checking stderr.

src/rt/rust_builtin.cpp

@@ -17,6 +17,7 @@
 #include "sync/timer.h"
 #include "sync/rust_thread.h"
 #include "rust_abi.h"
+#include "vg/valgrind.h"
 
 #include <time.h>
 
@@ -930,6 +931,11 @@ rust_begin_unwind(uintptr_t token) {
 #endif
 }
 
+extern "C" CDECL uintptr_t
+rust_running_on_valgrind() {
+    return RUNNING_ON_VALGRIND;
+}
+
 //
 // Local Variables:
 // mode: C++