btrace, linux: add perf event buffer abstraction

btrace, linux: add perf event buffer abstraction

Collect perf event buffer related fields from btrace_target_info into
a new struct perf_event_buffer.  Update functions that operated on the
buffer to take a struct perf_event_buffer pointer rather than a
btrace_target_info pointer.

2015-02-09  Markus Metzger  <markus.t.metzger@intel.com>

	* nat/linux-btrace.h (perf_event_buffer): New.
	(btrace_target_info) <buffer, size, data_head>: Replace with ...
	<bts>: ... this.
	* nat/linux-btrace.c (perf_event_header, perf_event_mmap_size)
	(perf_event_buffer_size, perf_event_buffer_begin)
	(perf_event_buffer_end, linux_btrace_has_changed): Removed.
	Updated users.
	(perf_event_new_data): New.