Constify get_exec_file

I noticed that get_exec_file could return a "const char *".  This
patch implements this change.

I couldn't build all the code -- but I did build Linux native and a
mingw cross.

Consequently, the NTO code has a hack, where it casts away const.  I
think this can be removed, but that required more work there, and
since I couldn't compile it, I felt it best not to try.

Let me know what you think.

gdb/ChangeLog
2019-12-16  Tom Tromey  <tromey@adacore.com>

	* windows-nat.c (windows_nat_target::attach): Update.
	* remote.c (extended_remote_target::attach): Update.
	* procfs.c (procfs_target::attach): Update.
	* nto-procfs.c (nto_procfs_target::attach): Update.
	(nto_procfs_target::create_inferior): Update.
	* inf-ptrace.c (inf_ptrace_target::attach): Update.
	* gnu-nat.c (gnu_nat_target::attach): Update.
	(gnu_nat_target::detach): Update.
	* darwin-nat.c (darwin_nat_target::attach): Update.
	* corefile.c (get_exec_file): Constify result.  Remove extraneous
	return.
	* bsd-kvm.c (bsd_kvm_target_open): Update.
	* gdbsupport/common-inferior.h (get_exec_file): Constify result.

gdb/gdbserver/ChangeLog
2019-12-16  Tom Tromey  <tromey@adacore.com>

	* server.c (get_exec_file): Constify result.

Change-Id: I29c60f7313a7def0dcb290ff0c2a4c1dea4f981f