Better Libdl.dlopen error when using non-standard extension (#46998)

When trying to dlopen a file with non-standard extension (e.g. `foo.so`
instead of `foo.dylib` when running on macOS), if this failed (e.g.
because of a reference to an undefined symbol), then instead of printing
the error message returned by `dlerror` with a helpful notice what
went wrong, a message indicating something to the effect that
"foo.so.dylib was not found" was shown, which was not helpful at all.

To get the actual helpful error message, add a check so that when dlopen
fails for a file that actually exists, we don't retry loading from a
file with the standard extension attached, which might not even exist;
instead we just give up.

This matches what is already being done for relative paths. This patch
simply copies the relevant check to also be applied to the case dealing
with absolute paths.

src/dlload.c

@@ -351,6 +351,10 @@ JL_DLLEXPORT void *jl_load_dynamic_library(const char *modname, unsigned flags,
 #ifdef _OS_WINDOWS_
         err = GetLastError();
         break; // LoadLibrary already tested the rest
+#else
+        // bail out and show the error if file actually exists
+        if (jl_stat(path, (char*)&stbuf) == 0)
+            break;
 #endif
     }