Share target_wait prototype between GDB and gdbserver

Share target_wait prototype between GDB and gdbserver

This commit moves the target_wait prototype from the GDB-specific
target.h header to the common target/target.h header.  Then, it
creates a compatible implementation of target_wait on gdbserver using
the_target->wait, and adjusts the (only) caller (mywait function).

Pretty straightforward, no regressions introduced.

gdb/gdbserver/ChangeLog:
2016-09-01  Sergio Durigan Junior  <sergiodj@redhat.com>

	* target.c (mywait): Call target_wait instead of
	the_target->wait.
	(target_wait): New function.

gdb/ChangeLog:
2016-09-01  Sergio Durigan Junior  <sergiodj@redhat.com>

	* target.c (target_wait): Mention that the function's prototype
	can be found at target/target.h.
	* target.h (target_wait): Move prototype from here...
	* target/target.h (target_wait): ... to here.