syscall: remove references to SYS_syscall on openbsd

syscall: remove references to SYS_syscall on openbsd

OpenBSD 7.5 no longer has a syscall symbol in libc. This will
typically result in external linking failures since the syscall
package still has a reference to an undefined `syscall' symbol.
Remove these references and return ENOSYS if syscall.Syscall* or
syscall.RawSyscall* are used for a system call number that does not
currently have an internal remapping.

Fixes #63900

Change-Id: Ic757bf8872ad98a92dd5b34cf58312c32fbc9a96
Reviewed-on: https://go-review.googlesource.com/c/go/+/582257
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>