std: Mark rust_get_num_cpus as dllexport

This function is imported across the DLL boundary by the libtest dynamic
library, so it has to be marked as dllexport somehow, and for now this is done
with an attribute on the function specifically.

src/rt/rust_builtin.c

@@ -45,6 +45,12 @@
 #include "valgrind/valgrind.h"
 #endif
 
+#if defined(_MSC_VER)
+# define RUST_BUILTIN_API __declspec(dllexport)
+#else
+# define RUST_BUILTIN_API
+#endif
+
 #ifndef _WIN32
 char*
 rust_list_dir_val(struct dirent* entry_ptr) {
@@ -129,6 +135,7 @@ get_num_cpus() {
 }
 #endif
 
+RUST_BUILTIN_API
 uintptr_t
 rust_get_num_cpus() {
     return get_num_cpus();