Revert "[MC] Remove isThumbFunc special case"

This reverts commit 8ee6c0e.

The untested special case is used as a workaround that we don't force emitting
R_ARM_REL32 relocations like GNU assembler's arm port: `TC_FORCE_RELOCATION_SUB_SAME`.
We shall investigate how to emit R_ARM_REL32.

llvm/lib/MC/MCExpr.cpp

@@ -606,6 +606,11 @@ static void AttemptToFoldSymbolOffsetDifference(
     return;
 
   auto FinalizeFolding = [&]() {
+    // Pointers to Thumb symbols need to have their low-bit set to allow
+    // for interworking.
+    if (Asm->isThumbFunc(&SA))
+      Addend |= 1;
+
     // If symbol is labeled as micromips, we set low-bit to ensure
     // correct offset in .gcc_except_table
     if (Asm->getBackend().isMicroMips(&SA))