btrace: split record_btrace_step_thread

btrace: split record_btrace_step_thread

The code for BTHR_STEP and BTHR_CONT is fairly similar.  Extract the common
parts into a new function record_btrace_single_step_forward.  The function
returns TARGET_WAITKIND_SPURIOUS to indicate that the single-step completed
without triggering a trap.

Same for BTHR_RSTEP and BTHR_RCONT.

gdb/
	* record-btrace.c (btrace_step_spurious)
	(record_btrace_single_step_forward)
	(record_btrace_single_step_backward): New.
	(record_btrace_step_thread): Call record_btrace_single_step_forward
	and record_btrace_single_step_backward.