Extend the mmap control page with fields so that userspace can compute
time deltas relative to the provided time fields.

Currently only implemented for x86 with constant and nonstop TSC.
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Cc: Arun Sharma <asharma@fb.com>
Link: http://lkml.kernel.org/n/tip-3u1jucza77j3wuvs0x2bic0f@git.kernel.orgSigned-off-by: NIngo Molnar <mingo@elte.hu>