gdbserver/Linux: Simplify stepping past program breakpoint a little

gdbserver/Linux: Simplify stepping past program breakpoint a little

.decr_pc_after_break is never higher than .breakpoint_len, so use
.breakpoint_len directly.  Based on idea from Yao here:
https://sourceware.org/ml/gdb-patches/2015-02/msg00689.html

gdb/gdbserver/ChangeLog:
2015-02-26  Pedro Alves  <palves@redhat.com>

	* linux-low.c (linux_wait_1): When incrementing the PC past a
	program breakpoint always use the_low_target.breakpoint_len as
	increment, rather than the maximum between that and
	the_low_target.decr_pc_after_break.