x86: adjust which Dwarf2 register numbers to use

x86: adjust which Dwarf2 register numbers to use

Consumers can't know which execution mode is in effect for a certain
piece of code; they can only go from object file properties. Hence which
register numbers to encode ought to depend solely on object file type.

In tc_x86_frame_initial_instructions() do away with parsing a register
name: We have a symbolic constant already for the 64-bit case, and the
32-bit number isn't going to change either. Said constant's definition
needs moving, though, to be available also for non-ELF. While moving
also adjust the comment to clarify that it's applicable to 64-bit mode
only.