mt: eliminate vector of structures with pointers (bad for libbusybox).

mt: eliminate vector of structures with pointers (bad for libbusybox).
It's a win for static build too:

function                                             old     new   delta
opcode_name                                            -     213    +213
opcode_value                                           -      68     +68
mt_main                                              281     256     -25
opcodes                                              280       -    -280
------------------------------------------------------------------------------
(add/remove: 2/1 grow/shrink: 0/1 up/down: 281/-305)          Total: -24 bytes
   text    data     bss     dec     hex filename
 767403     974    9420  777797   bde45 busybox_old
 767224     974    9420  777618   bdd92 busybox_unstripped