Revert "syscall: define Syscall in terms of RawSyscall on linux"

Revert "syscall: define Syscall in terms of RawSyscall on linux"

This reverts CL 388477, which breaks cmd/go
TestScript/cover_pkgall_runtime.

For #51087.
For #52472.

Change-Id: Id58af419a889281f15df2471c58fece011fcffbc
Reviewed-on: https://go-review.googlesource.com/c/go/+/401636
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>