Add default_breakpoint_from_pc

This patch adds the default implementation of gdbarch breakpoint_from_pc,
which is,

const gdb_byte *
default_breakpoint_from_pc (struct gdbarch *gdbarch, CORE_ADDR *pcptr,
                           int *lenptr)
{
  int kind = gdbarch_breakpoint_kind_from_pc (gdbarch, pcptr);

  return gdbarch_sw_breakpoint_from_kind (gdbarch, kind, lenptr);
}

so gdbarch can only defines sw_breakpoint_from_kind and
breakpoint_kind_from_pc.

gdb:

2016-11-03  Yao Qi  <yao.qi@linaro.org>

	* arch-utils.c (default_breakpoint_from_pc): New function.
	* arch-utils.h (GDBARCH_BREAKPOINT_FROM_PC): Remove.
	(GDBARCH_BREAKPOINT_MANIPULATION): Don't use
	GDBARCH_BREAKPOINT_FROM_PC.
	(SET_GDBARCH_BREAKPOINT_MANIPULATION): Don't call
	set_gdbarch_breakpoint_from_pc.
	(default_breakpoint_from_pc): Remove declaration.
	* gdbarch.sh (breakpoint_from_pc): Add its default implementation.
	* gdbarch.c, gdbarch.h: Regenerate.
	* arm-tdep.c: Don't use GDBARCH_BREAKPOINT_FROM_PC.
	* arc-tdep.c, bfin-tdep.c, cris-tdep.c, iq2000-tdep.c: Likewise.
	* m32r-tdep.c, mips-tdep.c, mt-tdep.c: Likewise.
	* nios2-tdep.c, score-tdep.c, sh-tdep.c: Likewise.
	* sh64-tdep.c, tic6x-tdep.c, v850-tdep.c, xtensa-tdep.c: Likewise.