[arm][arm64] Improve CountTrailingZero implementation.

src/arm/assembler-arm.h

Index: src/arm/assembler-arm.h
diff --git a/src/arm/assembler-arm.h b/src/arm/assembler-arm.h
index 5f34ed6c75911c37184d03a3f4bbc0272dceecae..6c38d7d33dad97f54a63885e7253f2f392878807 100644
--- a/src/arm/assembler-arm.h
+++ b/src/arm/assembler-arm.h
@@ -950,6 +950,9 @@ class Assembler : public AssemblerBase {
   void uxtah(Register dst, Register src1, Register src2, int rotate = 0,
              Condition cond = al);
 
+  // Reverse the bits in a register.
+  void rbit(Register dst, Register src, Condition cond = al);
+
   // Status register access instructions
 
   void mrs(Register dst, SRegister s, Condition cond = al);