Fix internal warning when "gdb -p xxx"

ps -e | grep a.out
28886 pts/12   00:00:00 a.out
gdb -p 28886
Loaded symbols for /lib64/ld-linux-x86-64.so.2
0x0000003b0ccbc970 in __nanosleep_nocancel () from /lib64/libc.so.6
../../binutils-gdb/gdb/cleanups.c:265: internal-warning: restore_my_cleanups has found a stale cleanup
A problem internal to GDB has been detected,
further debugging may prove unreliable.
Quit this debugging session? (y or n)

The backtrace of this issue:
(gdb) bt
    file=0x8b0c10 "s' failed.", line=265, fmt=0x8b0c38 "nutils-gdb/gdb/cleanups.c",
    ap=0x7fff803e3ed8) at ../../binutils-gdb/gdb/utils.c:748
    fmt=0x8b0c38 "nutils-gdb/gdb/cleanups.c", ap=0x7fff803e3ed8)
    at ../../binutils-gdb/gdb/utils.c:799
    string=0x8b0c38 "nutils-gdb/gdb/cleanups.c") at ../../binutils-gdb/gdb/utils.c:809
    at ../../binutils-gdb/gdb/cleanups.c:265
    at ../../binutils-gdb/gdb/cleanups.c:276
    at ../../binutils-gdb/gdb/exceptions.c:142
    at ../../binutils-gdb/gdb/exceptions.c:203
    command=0x5d5fb8 <attach_command_continuation_free_args+18>, arg=0x7fff803e525b "2914",
    from_tty=1, mask=RETURN_MASK_ALL) at ../../binutils-gdb/gdb/exceptions.c:549
---Type <return> to continue, or q <return> to quit---
    func_args=0x7fff803e4280, errstring=0x8cf2e4 "/local/bin", mask=RETURN_MASK_ALL)
    at ../../binutils-gdb/gdb/exceptions.c:522

This is a new issue.  It is introduced by commit https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=8bc2fe488957946d2cdccda3ce8d4f39e4003ea0
It removed the discard_cleanups (back_to) inside attach_command.
Then restore_my_cleanups will throw a internal_warning.

https://sourceware.org/ml/gdb-patches/2014-03/msg00374.html

2014-03-21  Hui Zhu  <hui@codesourcery.com>
	    Pedro Alves  <palves@redhat.com>

	* darwin-nat.c (darwin_pid_to_exec_file): Change xmalloc to
	static buffer.
	* fbsd-nat.c (fbsd_pid_to_exec_file): Ditto.
	* linux-nat.c (linux_child_pid_to_exec_file): Ditto.
	* nbsd-nat.c (nbsd_pid_to_exec_file): Ditto.