[gdb/testsuite] Fix gdb.tui/basic.exp on native-extended-gdbserver

With test-case gdb.tui/basic.exp and target board native-extended-gdbserver I
get:
...
status line: '<reverse:1>extended-r No process (asm) In:                                    L??   PC: ?? <reverse:0>'
FAIL: gdb.tui/basic.exp: status window: reverse
...
because the regexp:
...
gdb_assert { [regexp "^<.*>exec" $status] == 1 } \
...
tries to match:
...
status line: '<reverse:1>exec No process (asm) In:                                          L??   PC: ?? <reverse:0>'
...

Fix this by updating the regexp to allow both exec and extended-r.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>