x86: shrink opcode sets table

x86: shrink opcode sets table

Have i386-gen produce merely the offsets into i386_optab[]. Besides
allowing to shrink the table even on 32-bit builds, this results in
removing a level of indirection from the frequently accessed
current_templates, in return for adding a level of indirection when
looking up mnemonics (commonly happening just once per insn). Plus for
PIE builds of gas it also reduces the number of relocations by about two
thousand. Finally a somewhat ugly static variable can also be eliminated
from i386_displacement().