[gdb/symtab] Fix too many symbols in gdbpy_lookup_static_symbols

[gdb/symtab] Fix too many symbols in gdbpy_lookup_static_symbols

When running test-case gdb.python/py-symbol.exp with target board
cc-with-dwz-m, we run into:
...
(gdb) python print (len (gdb.lookup_static_symbols ('rr')))^M
4^M
(gdb) FAIL: gdb.python/py-symbol.exp: \
  print (len (gdb.lookup_static_symbols ('rr')))
...
while with target board unix we have instead:
...
(gdb) python print (len (gdb.lookup_static_symbols ('rr')))^M
2^M
(gdb) PASS: gdb.python/py-symbol.exp: \
  print (len (gdb.lookup_static_symbols ('rr')))
...

The problem is that the loop in gdbpy_lookup_static_symbols loops over compunits
representing both CUs and PUs:
...
 	  for (compunit_symtab *cust : objfile->compunits ())
...

When doing a lookup on a PU, the user link is followed until we end up at a CU,
and the lookup is done in that CU.

In other words, when doing a lookup in the loop for a PU we duplicate the
lookup for a CU that is already handled by the loop.

Fix this by skipping PUs in the loop in gdb.lookup_static_symbols.

Tested on x86_64-linux.

PR symtab/25261
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=25261