Allow building the `singlejar` tool on OpenBSD.

Without this change, the build fails with the following error. Thus, building deploy JARs is impossible.

```
ld: error: undefined symbol: ceilf
>>> referenced by output_jar.cc
```
PiperOrigin-RevId: 319890175
diff --git a/tools/jdk/BUILD.java_tools b/tools/jdk/BUILD.java_tools
index a33e6ff..2315eec 100644
--- a/tools/jdk/BUILD.java_tools
+++ b/tools/jdk/BUILD.java_tools
@@ -338,6 +338,12 @@
     visibility = ["//visibility:public"],
 )
 
+config_setting(
+    name = "openbsd",
+    values = {"cpu": "openbsd"},
+    visibility = ["//visibility:public"],
+)
+
 alias(
     name = "singlejar",
     actual = select({
@@ -633,6 +639,7 @@
     copts = SUPRESSED_WARNINGS,
     linkopts = select({
         ":freebsd": ["-lm"],
+        ":openbsd": ["-lm"],
         "//conditions:default": [],
     }),
     linkstatic = 1,
@@ -655,6 +662,7 @@
     copts = SUPRESSED_WARNINGS,
     linkopts = select({
         ":freebsd": ["-lm"],
+        ":openbsd": ["-lm"],
         "//conditions:default": [],
     }),
     linkstatic = 1,