Disable DL_LOAD_PATH prepending for @-paths on Darwin (#42721)

* Disable `DL_LOAD_PATH` prepending for `@`-paths on Darwin

Many thanks to Randy Rucker from Apple for pointing out that we were
technically relying on undefined behavior in `dyld` for loading things
via `jl_dlopen("@loader_path/@rpath/libfoo.dylib")`, due to the
composition of `dlopen("@rpath/libfoo.dylib")` in our Julia code, and
the `DL_LOAD_PATH` prepending we do in `jl_load_dynamic_library()`.

This PR uses a slightly modified version of a patch emailed to me by
Randy, and should solve
JuliaLang/Downloads.jl#149 on macOS Monterey
where the undefined behavior in `dyld` has changed.

* Apply suggestions from code review

Co-authored-by: Jameson Nash <vtjnash@gmail.com>

* Use `[]` instead of `*`

Co-authored-by: Jameson Nash <vtjnash@gmail.com>
(cherry picked from commit 76c2431)

src/dlload.c

@@ -163,6 +163,7 @@ JL_DLLEXPORT void *jl_load_dynamic_library(const char *modname, unsigned flags,
     uv_stat_t stbuf;
     void *handle;
     int abspath;
+    int is_atpath;
     // number of extensions to try — if modname already ends with the
     // standard extension, then we don't try adding additional extensions
     int n_extensions = endswith_extension(modname) ? 1 : N_EXTENSIONS;
@@ -188,16 +189,30 @@ JL_DLLEXPORT void *jl_load_dynamic_library(const char *modname, unsigned flags,
     }
 
     abspath = isabspath(modname);
+    is_atpath = 0;
+
+    // Detect if our `modname` is something like `@rpath/libfoo.dylib`
+#ifdef _OS_DARWIN_
+    size_t nameLen = strlen(modname);
+    const char *const atPaths[] = {"@executable_path/", "@loader_path/", "@rpath/"};
+    for (i = 0; i < sizeof(atPaths)/sizeof(char*); ++i) {
+        size_t atLen = strlen(atPaths[i]);
+        if (nameLen >= atLen && 0 == strncmp(modname, atPaths[i], atLen)) {
+            is_atpath = 1;
+        }
+    }
+#endif
 
     /*
       this branch permutes all base paths in DL_LOAD_PATH with all extensions
       note: skip when !jl_base_module to avoid UndefVarError(:DL_LOAD_PATH),
             and also skip for absolute paths
+            and also skip for `@`-paths on macOS
       We also do simple string replacement here for elements starting with `@executable_path/`.
       While these exist as OS concepts on Darwin, we want to use them on other platforms
       such as Windows, so we emulate them here.
     */
-    if (!abspath && jl_base_module != NULL) {
+    if (!abspath && !is_atpath && jl_base_module != NULL) {
         jl_binding_t *b = jl_get_module_binding(jl_base_module, jl_symbol("DL_LOAD_PATH"));
         jl_array_t *DL_LOAD_PATH = (jl_array_t*)(b ? b->value : NULL);
         if (DL_LOAD_PATH != NULL) {