as: AArch64 Mapping Symbols

 
 9.1.7 Mapping Symbols
 ---------------------
 
 The AArch64 ELF specification requires that special symbols be inserted
 into object files to mark certain features:
 
 '$x'
      At the start of a region of code containing AArch64 instructions.
 
 '$d'
      At the start of a region of data.