runtime: use entersyscall in syscall_syscallX on Darwin

CL 197938 changed syscall* functions to call entersyscall, instead
of entersyscallblock. It missed syscall_syscallX, probably because
it was in sys_darwin_64.go, not sys_darwin.go like others. Change
that one as well.

Found during the review of CL 270380 (thanks Joel).

Change-Id: I0884fc766703f555a3895be332dccfa7d2431374
Reviewed-on: https://go-review.googlesource.com/c/go/+/286435
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/sys_darwin.go

@@ -27,7 +27,7 @@ func syscall()
 //go:nosplit
 //go:cgo_unsafe_args
 func syscall_syscallX(fn, a1, a2, a3 uintptr) (r1, r2, err uintptr) {
-	entersyscallblock()
+	entersyscall()
 	libcCall(unsafe.Pointer(funcPC(syscallX)), unsafe.Pointer(&fn))
 	exitsyscall()
 	return