Refactor default_breakpoint_kind_from_pc to be used by all targets in GDBServer.

Refactor default_breakpoint_kind_from_pc to be used by all targets in GDBServer.

This patch moves default_breakpoint_kind_from_pc to target.c and creates a macro
so that all targets can easily use it.

This allows the breakpoint_kind_from_pc operation to be left unimplemented in
targets that do not need it.

This is preparation to fix the win32/nto/spu build that was broken by this
patch: https://sourceware.org/ml/gdb-patches/2015-10/msg00369.html

No regression on Ubuntu 14.04 x86-64 with gdbserver-{native-extended}

gdb/gdbserver/ChangeLog:

	* linux-low.c (default_breakpoint_kind_from_pc): Move to target.c.
	* mem-break.c (set_breakpoint_at): Use target_breakpoint_kind_from_pc.
	* target.c (default_breakpoint_kind_from_pc): Moved from linux-low.c
	* target.h (target_breakpoint_kind_from_pc): New macro.