Rename "master" branch to "main"

Update all links accordingly. Also update links to other repositories
whose branches have renamed.

The references to repositories whose branches have not renamed should be
referencing specific commits anyway, so reference those specific commits
by hash.

samples/basic/src/main.c

@@ -61,7 +61,7 @@ int main(int argc, char *argv_main[])
     // Note: the array must be static defined since runtime
     //            will keep it after registration
     // For the function signature specifications, goto the link:
-    // https://github.com/bytecodealliance/wasm-micro-runtime/blob/master/doc/export_native_api.md
+    // https://github.com/bytecodealliance/wasm-micro-runtime/blob/main/doc/export_native_api.md
 
     static NativeSymbol native_symbols[] =
     {