dlopen: Fix system libraries searchpath (#441)

The canonical path for native system libraries is `/system/lib` for 32bits and `/system/lib64`
for 64bits (c.f., https://source.android.com/docs/core/permissions/namespaces_libraries).

This was completely ignored, leading to explicit ffi.load calls on system libraries (e.g., libc for threading) attempting to load a 32bits variant on a 64bits ABI...

assets/android.lua

@@ -2654,9 +2654,10 @@ local function run(android_app_state)
 
     -- load the dlopen() implementation
     android.dl = require("dl")
+    android.dl.system_libdir = ffi.abi("64bit") and "/system/lib64" or "/system/lib"
     android.dl.library_path = android.nativeLibraryDir..":"..
         android.dir..":"..android.dir.."/libs:"..
-        "/lib:/system/lib:/lib/lib?.so:/system/lib/lib?.so"
+        string.gsub("@:@/lib?.so", "@", android.dl.system_libdir)
 
     -- register the dependency lib loader
     table.insert(package.loaders, 3, android.deplib_loader)

assets/dl.lua

@@ -54,6 +54,9 @@ end
 local dl = {
     -- set this to search in certain directories
     library_path = '/lib/?;/usr/lib/?;/usr/local/lib/?',
+    -- set this to the directory of system libraries
+    -- (to be ignored when loading dependencies).
+    system_libdir = nil,
 }
 
 local function sys_dlopen(library, global, padding)
@@ -114,7 +117,7 @@ function dl.dlopen(library, load_func, depth)
             -- libvulkan, and libz
             -- c.f., https://android.googlesource.com/platform/bionic/+/master/android-changes-for-ndk-developers.md#private-api-enforced-for-api-level-24
             -- Our current code should *never* hit any private system libs, so, this is basically overkill ;).
-            if depth > 0 and (pspec == "/system/lib" or library == "libdl.so") then
+            if depth > 0 and (pspec == dl.system_libdir or library == "libdl.so") then
                 -- depth > 0 to allow explicitly loading a system lib
                 -- (because this might have genuine use cases, as some early API levels do not put DT_NEEDED libraries into the global namespace)
                 -- pspec to reject system libs