tcg: Put target helper data into an array.

One call inside of a loop to tcg_register_helper instead of hundreds
of sequential calls.

Presumably more icache and branch prediction friendly; resulting binary
size mostly unchanged on x86_64, as we're trading 32-bit rip-relative
references in .text for full 64-bit pointers in .rodata.

Signed-off-by: Richard Henderson <rth@twiddle.net>
