syscall: define Syscall6 in terms of RawSyscall6 on linux

syscall: define Syscall6 in terms of RawSyscall6 on linux

This is an exact copy of CL 388478 after fixing #52472 in CL 401654.

For #51087
For #52472

Change-Id: I6c6bd7ddcab1512c682e6b44f61c7bcde97f5c58
Reviewed-on: https://go-review.googlesource.com/c/go/+/401655
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>