[PATCH] [testsuite] Fix in board_file native-gdbserver.exp

Hi,
When I use following statement in test case with native-gdbserver.exp board_file,
I got one error,

  remote_exec target "sh -c { \[ -f /bin/ls \] }"

  WARNING: program timed out.
  FAIL: gdb.base/foo.exp: exists /bin/ls

Examine the verbose log shows that dejagnu thinks native-gdbserver is a
remote target, and will try to use remote method, rsh for example, to
access files,

 Executing on target: sh -c { [ -f /bin/ls ] }    (timeout = 300)
 calling is_remote target
 board_info build name
 getting qiyao name
 board_info host name
 getting qiyao name
 calling is_remote native-gdbserver
 board_info build name
 getting qiyao name
 board_info host name
 getting qiyao name
 board_info native-gdbserver exists isremote
 board_info native-gdbserver isremote
 getting native-gdbserver isremote
 board is native-gdbserver, isremote is 1

This patch add new proc ${board}_exec to wrap remote_exec with setting