x86: change type of Dwarf2 register numbers in register table

Already the %bnd<N> registers used numbers beyond 127, and eGPR ones are
all out of reach for "signed char", at least when CHAR_BITS=8. Switch to
"unsigned char", covering appropriately in places where the value
returned for "none" actually matters (in tc_x86_parse_to_dw2regnum()
this is actually achieved by altering how X_op is set).