Ignore /SYM64/ pseudo member inar

I believe this pseudo member is relatively new to support archive
files larger than 4 GiB. I'm not sure if mold can handle such large
archives, but first of all, mold crashes when an archive with this
pseudo member is given.

This commit fixes the crash bug.

I don't know how to create an archive with this pseudo member, so
this commit does not have a testcase.

Reported by sina-ht at #269.

archive-file.h

@@ -82,7 +82,8 @@ read_fat_archive_members(C &ctx, MappedFile<C> *mf) {
     }
 
     // Skip if symbol table
-    if (memcmp(hdr.ar_name, "/ ", 2) == 0)
+    if (memcmp(hdr.ar_name, "/ ", 2) == 0 ||
+        memcmp(hdr.ar_name, "/SYM64/ ", 8) == 0)
       continue;
 
     // Read the name field