chore: Use Bazel's built-in constraint values

The toktok-stack constraints are thin wrappers around the
bazelbuild/platforms ones, but using the Bazel ones makes depending on
jvm-toxcore-c more convenient.

BUILD.bazel

@@ -10,10 +10,10 @@ project()
 genrule(
     name = "copy_link_jni_md_header",
     srcs = select({
-        "//tools/config:freebsd": ["@bazel_tools//tools/jdk:jni_md_header-freebsd"],
-        "//tools/config:linux": ["@bazel_tools//tools/jdk:jni_md_header-linux"],
-        "//tools/config:osx": ["@bazel_tools//tools/jdk:jni_md_header-darwin"],
-        "//tools/config:windows": ["@bazel_tools//tools/jdk:jni_md_header-windows"],
+        "@platforms//os:freebsd": ["@bazel_tools//tools/jdk:jni_md_header-freebsd"],
+        "@platforms//os:linux": ["@bazel_tools//tools/jdk:jni_md_header-linux"],
+        "@platforms//os:osx": ["@bazel_tools//tools/jdk:jni_md_header-darwin"],
+        "@platforms//os:windows": ["@bazel_tools//tools/jdk:jni_md_header-windows"],
     }),
     outs = ["lib/src/jvmMain/cpp/jni_md.h"],
     cmd = "cp -f $< $@",
@@ -58,10 +58,10 @@ cc_binary(
         "lib/src/jvmMain/proto",
     ],
     linkopts = select({
-        "//tools/config:freebsd": ["-Wl,--version-script,$(location lib/src/jvmMain/cpp/libtox4j-c.ld)"],
-        "//tools/config:linux": ["-Wl,--version-script,$(location lib/src/jvmMain/cpp/libtox4j-c.ld)"],
-        "//tools/config:osx": [],
-        "//tools/config:windows": [],
+        "@platforms//os:freebsd": ["-Wl,--version-script,$(location lib/src/jvmMain/cpp/libtox4j-c.ld)"],
+        "@platforms//os:linux": ["-Wl,--version-script,$(location lib/src/jvmMain/cpp/libtox4j-c.ld)"],
+        "@platforms//os:osx": [],
+        "@platforms//os:windows": [],
     }),
     linkshared = True,
     visibility = ["//visibility:public"],