Replace some $ARCH_{get,set}_pc with linux_{get,set}_pc_64bit

Replace some $ARCH_{get,set}_pc with linux_{get,set}_pc_64bit

This patch is the follow-up of
https://sourceware.org/ml/gdb-patches/2016-01/msg00164.html to provide
linux_{get,set}_pc_64bit functions.

Rebuild GDBserver with tilegx-linux-gcc.  Not tested.

I think about pc in Tile-GX a little bit.  Looks current Tile-GX
supports debugging 32-bit program (multi-arch), but PC is always
64-bit.  See this thread
https://sourceware.org/ml/gdb-patches/2013-02/msg00113.html
and GDBserver reads PC as 64-bit through ptrace.  However, if
the inferior is 32-bit, the PC in the target description and
regcache is 32-bit, so only 32-bit contents are sent back GDB.
Anyway, Tile-GX GDBserver may have some problems here, but this
patch doesn't change anything.

gdb/gdbserver:

2016-01-18  Yao Qi  <yao.qi@linaro.org>

	* linux-low.c (linux_set_pc_64bit): New function.
	(linux_get_pc_64bit): New function.
	* linux-low.h (linux_set_pc_64bit, linux_get_pc_64bit):
	Declare.
	* linux-sparc-low.c (debug_threads): Remove declaration.
	(sparc_get_pc): Remove.
	(the_low_target): Use linux_get_pc_64bit instead of
	sparc_get_pc.
	* linux-tile-low.c (tile_get_pc, tile_set_pc): Remove.
	(the_low_target): Use linux_get_pc_64bit and
	linux_set_pc_64bit.