unix: consistently use tabs in //sys and //sysnb lines

There is some inconsistent use of tabs and spaces in the //sys and
//sysnb lines. However, the large majority is using tabs, so use these
consistently.

Also change the regexp in mksyscall*.go to only accept tabs going
forward.

Change-Id: I78944e88b5fec11c1bd1d103cea894bc5109da56
Reviewed-on: https://go-review.googlesource.com/c/sys/+/283596
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_linux_arm64.go

@@ -155,7 +155,7 @@ func Pipe(p []int) (err error) {
 	return
 }
 
-//sysnb pipe2(p *[2]_C_int, flags int) (err error)
+//sysnb	pipe2(p *[2]_C_int, flags int) (err error)
 
 func Pipe2(p []int, flags int) (err error) {
 	if len(p) != 2 {