unix: define PerfBitWriteBackward

Change-Id: I0b95006039b4efcd0094ba96281100abeafc993e
GitHub-Last-Rev: 43383aa
GitHub-Pull-Request: #149
Reviewed-on: https://go-review.googlesource.com/c/sys/+/473135
Reviewed-by: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Tobias Klauser <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>

Author: Francis Laniel

unix/linux/types.go

@@ -1115,6 +1115,7 @@ const (
 	PerfBitCommExec                      = CBitFieldMaskBit24
 	PerfBitUseClockID                    = CBitFieldMaskBit25
 	PerfBitContextSwitch                 = CBitFieldMaskBit26
+	PerfBitWriteBackward                 = CBitFieldMaskBit27
 )
 
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