macos: Create symtab

macos: Create symtab

this creates a proper LC_SYMTAB, with reasonable entries.  It's
not sorted, so not usable for LC_DYSYMTAB.  But 'nm -x -no-sort'
allows to see us some useful info.

This also relocates sections and symbols, so now this example
works as well (i.e. read access to static local data):

% cat simple2.c
static int loc = 42;
int main(void)
{
  return loc - 42;
}