convert current_target to be a pointer

This converts current_target to be a pointer rather than a struct.
This is more efficient when we move to multi-target, because then we
can switch targets by simply changing the pointer.

I made this patch by running this:

perl -pi -e 's/&current_target/current_target/g; s/current_target\./current_target->/g;' *.[chyl] */*.[chyl]

... and then fixing up the result minorly:

* Updating the definition and declaration of current_target
* Fixing a "sizeof"
* Initializing current_target

Built and regtested on x86-64 Fedora 20.

2014-07-29  Tom Tromey  <tromey@redhat.com>

	* target.c (current_target): Now a pointer.
	(update_current_target): Use size of target_ops, not
	current_target.
	(initialize_targets): Initialize current_target.
	* target.h (current_target): Now a pointer.
	* auxv.c: Update.
	* avr-tdep.c: Update.
	* breakpoint.c: Update.
	* corefile.c: Update.
	* elfread.c: Update.
	* eval.c: Update.
	* frame.c: Update.
	* gnu-v3-abi.c: Update.
	* ia64-hpux-tdep.c: Update.
	* ia64-tdep.c: Update.
	* ia64-vms-tdep.c: Update.
	* infcall.c: Update.
	* infcmd.c: Update.
	* infrun.c: Update.
	* linespec.c: Update.
	* linux-nat.c: Update.
	* linux-tdep.c: Update.
	* linux-thread-db.c: Update.
	* mi/mi-main.c: Update.
	* minsyms.c: Update.
	* parse.c: Update.
	* ppc-linux-nat.c: Update.
	* ppc-linux-tdep.c: Update.
	* procfs.c: Update.
	* remote-m32r-sdi.c: Update.
	* remote.c: Update.
	* s390-linux-nat.c: Update.
	* s390-linux-tdep.c: Update.
	* sol-thread.c: Update.
	* solib-aix.c: Update.
	* solib-darwin.c: Update.
	* solib-dsbt.c: Update.
	* solib-ia64-hpux.c: Update.
	* solib-irix.c: Update.
	* solib-osf.c: Update.
	* solib-spu.c: Update.
	* solib-svr4.c: Update.
	* solib-target.c: Update.
	* solib.c: Update.
	* sparc-tdep.c: Update.
	* spu-tdep.c: Update.
	* symfile.c: Update.
	* target-descriptions.c: Update.
	* target-memory.c: Update.
	* target.c: Update.
	* target.h: Update.
	* tracepoint.c: Update.
	* valops.c: Update.
	* valprint.c: Update.
	* value.c: Update.
	* windows-tdep.c: Update.