* cygtls.cc (san::leave/x86_64): Implement.

	* cygtls.cc (san::leave/x86_64): Implement.
	* cygtls.h (class tls_pathbuf): Move counter variables into a union.
	Add 64 bit element _counters covering both counter variables to
	optimize save and restore operations.
	(class san/x86_64): Only store single 64 bit value.
	(san::san/x86_64): Implement.
	(san::leave/x86_64): Only declare here, as returns_twice function.
	Explain why.
	(class san/i686): Change type of _c_cnt and _w_cnt to uint32_t.
	(__try/x86_64): Move definition of __sebastian after the first memory
	barrier.  Drop __sebastian.setup call.