print-symbol-loading.exp: Allow libc symbols to be already loaded

print-symbol-loading.exp: Allow libc symbols to be already loaded

One consequence of changing libpthread_name_p() in solib.c to (also)
match libc is that the symbols for libc will now be loaded by
solib_add() in solib.c.  I think this is mostly harmless because
we'll likely want these symbols to be loaded anyway, but it did cause
two failures in gdb.base/print-symbol-loading.exp.

Specifically...

1)

sharedlibrary .*
(gdb) PASS: gdb.base/print-symbol-loading.exp: shlib off: load shared-lib

now looks like this:

sharedlibrary .*
Symbols already loaded for /lib64/libc.so.6
(gdb) PASS: gdb.base/print-symbol-loading.exp: shlib off: load shared-lib

2)

sharedlibrary .*
Loading symbols for shared libraries: .*
(gdb) PASS: gdb.base/print-symbol-loading.exp: shlib brief: load shared-lib

now looks like this:

sharedlibrary .*
Loading symbols for shared libraries: .*
Symbols already loaded for /lib64/libc.so.6
(gdb) PASS: gdb.base/print-symbol-loading.exp: shlib brief: load shared-lib

Fixing case #2 ended up being easier than #1.  #1 had been using
gdb_test_no_output to correctly match this no-output case.  I
ended up replacing it with gdb_test_multiple, matching the exact
expected output for each of the two now acceptable cases.

For case #2, I simply added an optional non-capturing group
for the potential new output.

gdb/testsuite/ChangeLog:

	* gdb.base/print-symbol-loading.exp (proc test_load_shlib):
	Allow "Symbols already loaded for..." messages.