Make "gnutarget" const

I noticed that gnutarget was not "const".  Since writing through this
pointer would probably be a bug, I think it ought to be.  This patch
makes the change.

gdb/ChangeLog
2020-03-04  Tom Tromey  <tom@tromey.com>

	* jit.c (bfd_open_from_target_memory): Make "target" const.
	* corefile.c (gnutarget): Now const.
	* gdbcore.h (gnutarget): Now const.
4 files changed