x86-64: Add vsyscall:emulate_vsyscall trace event

Vsyscall emulation is slow, so make it easy to track down.

Signed-off-by: Andy Lutomirski <luto@mit.edu>
Link: http://lkml.kernel.org/r/cdaad7da946a80b200df16647c1700db3e1171e9.1312378163.git.luto@mit.edu
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>