[C++] remote.c: Avoid enum arithmetic

Fixes:

  src/gdb/remote.c: In function ‘void remote_unpush_target()’:
  src/gdb/remote.c:4610:45: error: invalid conversion from ‘int’ to ‘strata’ [-fpermissive]
     pop_all_targets_above (process_stratum - 1);
					       ^
  In file included from src/gdb/inferior.h:38:0,
		   from src/gdb/remote.c:25:
  src/gdb/target.h:2299:13: error:   initializing argument 1 of ‘void pop_all_targets_above(strata)’ [-fpermissive]
   extern void pop_all_targets_above (enum strata above_stratum);
	       ^

I used to carry a patch in the C++ branch that just did:

 -  pop_all_targets_above (process_stratum - 1);
 +  pop_all_targets_above ((enum strata) (process_stratum - 1));

But then thought that maybe adding a routine that does exactly what we
need results in clearer code.  This is the result.

gdb/ChangeLog:
2015-11-19  Pedro Alves  <palves@redhat.com>

	* remote.c (remote_unpush_target): Use
	pop_all_targets_at_and_above instead of pop_all_targets_above.
	* target.c (unpush_target_and_assert): New function, factored out
	from ...
	(pop_all_targets_above): ... here.
	(pop_all_targets_at_and_above): New function.
	* target.h (pop_all_targets_at_and_above): Declare.