Skip an unnecessary layout pass for non-Android links

After fix_synthetic_symbols, we re-ran set_osec_offsets
unconditionally. That pass is needed only under
--pack-dyn-relocs=android, whose packed relocation format encodes
addends in variable-length SLEB128, so fixing synthetic-symbol
values can change the encoded size of .rela.dyn. No other mode
emits anything whose size depends on addends; in particular,
RELR's size depends only on relocation locations, which are fixed
by this point. Make the pass conditional on the Android mode.

On the Firefox release benchmark with 32 threads and --no-fork,
this shortens the link from 0.44 s to 0.41 s. Outputs are
byte-for-byte identical before and after for both a RELR workload
(Firefox) and a RELA workload (Chromium release).

Co-Authored-By: Claude Fable 5 <[email protected]>

src/main.cc

@@ -689,9 +689,12 @@ int mold_main(int argc, char **argv) {
 
   // Re-finalize layout. fix_synthetic_symbols above may have changed
   // addends for dynamic relocations referencing synthetic symbols, which
-  // can shift the encoded size of .rela.dyn under --pack-dyn-relocs=android.
-  // For other modes this is a cheap no-op convergence.
-  filesize = set_osec_offsets(ctx);
+  // can shift the encoded size of .rela.dyn under --pack-dyn-relocs=android
+  // because Android's packed format encodes addends in variable-length
+  // SLEB128. Other modes, including ordinary RELR, encode nothing whose
+  // size depends on addends, so they do not need this pass.
+  if (ctx.arg.pack_dyn_relocs_android)
+    filesize = set_osec_offsets(ctx);
 
   // At this point, both memory and file layouts are fixed.