runtime: simplify openbsd check in usesLibcall and mStackIsSystemAllocated

The openbsd/mips64 runtime code was removed in CL 649659.

For golang#61546

Change-Id: I03f16c3396baddb0ee9aa751dd6f699a835e7586
Reviewed-on: https://go-review.googlesource.com/c/go/+/700976
Auto-Submit: Tobias Klauser <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
Reviewed-by: Joel Sing <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: tklauser

src/runtime/proc.go

@@ -1816,10 +1816,8 @@ func startTheWorldWithSema(now int64, w worldStop) int64 {
 // via libcall.
 func usesLibcall() bool {
 	switch GOOS {
-	case "aix", "darwin", "illumos", "ios", "solaris", "windows":
+	case "aix", "darwin", "illumos", "ios", "openbsd", "solaris", "windows":
 		return true
-	case "openbsd":
-		return GOARCH != "mips64"
 	}
 	return false
 }
@@ -1828,10 +1826,8 @@ func usesLibcall() bool {
 // system-allocated stack.
 func mStackIsSystemAllocated() bool {
 	switch GOOS {
-	case "aix", "darwin", "plan9", "illumos", "ios", "solaris", "windows":
+	case "aix", "darwin", "plan9", "illumos", "ios", "openbsd", "solaris", "windows":
 		return true
-	case "openbsd":
-		return GOARCH != "mips64"
 	}
 	return false
 }