gdb/infrun: enable/disable thread events of all targets in stop_all_threads

gdb/infrun: enable/disable thread events of all targets in stop_all_threads

In stop_all_threads, the thread events of the current top target are
enabled at the beginning of the function and then disabled at the end
(at scope exit time).  Because there may be multiple targets whose
thread lists will be updated and whose threads are stopped,
enable/disable thread events for all targets.

This update caused a change in the annotations.  In particular, a
"frames-invalid" annotation is printed one more time due to switching
the current inferior.  Hence, gdb.base/annota1.exp and
gdb.cp/annota2.exp tests are also updated.

Regression-tested on X86_64 Linux using the default board file and the
native-extended-gdbserver board file.

gdb/ChangeLog:
2020-05-14  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* infrun.c (stop_all_threads): Enable/disable thread events of all
	targets.  Move a debug message denoting the end of the function
	into the SCOPED_EXIT block.

gdb/testsuite/ChangeLog:
2020-05-14  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* gdb.base/annota1.exp: Update the expected output.
	* gdb.cp/annota2.exp: Ditto.