LoongArch: Assign DWARF register numbers to register aliases

.cfi directives only support the use of register numbers and not
register names or aliases.

This commit adds support for 4 formats, for example:
  .cfi_offset r1, 8
  .cfi_offset ra, 8
  .cfi_offset $r1,8
  .cfi_offset $ra,8

The above .cfi directives are equivalent and all represent dwarf
register number 1.

Display register aliases as specified in the psABI during disassembly.