btrace, linux: use data_size and data_offset

btrace, linux: use data_size and data_offset

In struct perf_event_mmap_page there are new fields data_size and data_offset
that give the location of the perf_event data buffer relative to the mmap
page.  Use them if they are present.

gdb/
	* nat/linux-btrace.c (linux_enable_bts): Check for
	PERF_ATTR_SIZE_VER5.
	Check for data_offset and data_size fields.  Use them.