x86: instantiate i386_{op,reg}tab[] in gas instead of in libopcodes

Unlike many other architectures, x86 does not share an opcode table
between assembly and disassembly. Any consumer of libopcodes would only
ever access one of the two. Since gas is the only consumer of the
assembly data, move it there. While doing so mark respective entities
"static" in i386-gen (we may want to do away with i386_regtab_size
altogether).

This also shrinks the number of relocations to be processed for
libopcodes.so by about 30%.