6l: eliminate dead code, not just the symbols

6l: eliminate dead code, not just the symbols

editing the firstp list was ineffective,
because follow rebuilds it from the textp list.

the symbols for dead code were being dropped
from the binary but the code was all still there.

text for fmt.Printf("hello, world\n") drops
from 143945 to 128650.

R=r,ken
DELTA=22  (20 added, 0 deleted, 2 changed)
OCL=28255
CL=28290