x86: remove indirection from bx[] and di_si[]

x86: remove indirection from bx[] and di_si[]

The longest register name is 3 characters (plus a nul one), so using a
4- or 8-byte pointer to get at it is neither space nor time efficient.
Embed the names right into the array. For PIE this also slightly reduces
the number of base relocations in the final image.