[gdb/testsuite] Fix gdb.base/maint.exp with cc-with-gdb-index

[gdb/testsuite] Fix gdb.base/maint.exp with cc-with-gdb-index

With target board cc-with-gdb-index we run into:
...
FAIL: gdb.base/maint.exp: maint print statistics
...

The output that is checked is:
...
Statistics for 'maint':^M
  Number of "minimal" symbols read: 53^M
  Number of "full" symbols read: 40^M
  Number of "types" defined: 60^M
  Number of symbol tables: 7^M
  Number of symbol tables with line tables: 2^M
  Number of symbol tables with blockvectors: 2^M
  Number of read CUs: 2^M
  Number of unread CUs: 5^M
  Total memory used for objfile obstack: 20320^M
  Total memory used for BFD obstack: 4064^M
  Total memory used for string cache: 4064^M
...
and the regexp doesn't match because it expects the "Number of read/unread
CUs" lines in a different place.

Fix this by updating the regexp.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-08-01  Tom de Vries  <tdevries@suse.de>

	* gdb.base/maint.exp: Update "maint print statistics" to match
	output with target board cc-with-gdb-index.