Revert "syscall: define Syscall6 in terms of RawSyscall6 on linux"

Revert "syscall: define Syscall6 in terms of RawSyscall6 on linux"

This reverts CL 388478. Parent CL 388477 breaks cmd/go
TestScript/cover_pkgall_runtime.

For #51087.
For #52472.

Change-Id: Id5d5a4e138792cf130ecdcc6b996c8102d142a7e
Reviewed-on: https://go-review.googlesource.com/c/go/+/401635
Run-TryBot: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>