remote: call remote_check_symbols after attaching

remote: call remote_check_symbols after attaching

When debugging a remote bare-metal target with "target
extended-remote" + attach, GDB won't send a qSymbol packet to initiate
symbol lookup.  This happens because all the previous places in which
GDB might have done this are guarded by conditions that don't hold in
the said scenario: there are no shared libraries, no vsyscall page and
the binary file didn't change in the time passed between the "file"
and the "attach" commands.

To solve this problem remote_check_symbols is called in the
target_post_attach hook.

gdb/
2014-07-11  Adrian Sendroiu  <adrian.sendroiu@freescale.com>

	* remote.c (extended_remote_post_attach): New function.
	(init_extended_remote_ops): Install it as to_post_attach method.