(VMS_Initialized_Data_Size): Cache symbol values to reduce number of lookups

with S_GET_VALUE.  Skip debug symbols to avoid "a really nasty bug".  (From
Holger Teutsch, holger@botbso.rhein-main.de.)
(VMS_write_object_file): For "__vt.*" symbols, set S_GET_OTHER field.  (Also
from Holger Teutsch.)  Watch for a would-be register mask that spans frags.