build-logic: link sanitizer runtime for the fuzzer shared lib (#646)

commonLinuxLinkerArgs() carries -Wl,-z,defs, which forbids undefined
symbols in the output. The fuzzer objects are instrumented with
-fsanitize=address,undefined and therefore reference __asan_*/__ubsan_*,
but -fsanitize=address only links the sanitizer runtime into
*executables*, not shared libraries. So the fuzzer .so ends up with
undefined __asan_*/__ubsan_* and the link fails under -z defs on any box
where the fuzzer config is active (clang with libFuzzer, non-musl).

The asan config already solves this by linking the matching runtime
explicitly via locateLibasan (-lclang_rt.asan-<arch> on clang, which also
provides UBSan symbols; -lasan + -lubsan on gcc). Apply the same,
already-proven treatment to configureFuzzer: thread the detected compiler
through and add the runtime to the Linux linker args.

CI never hit this because its publish/build path passes
--exclude-task compileFuzzer and only assembles the release jar, so the
fuzzer is never linked there. It only bites a local build that links all
active configs (e.g. publishToMavenLocal on a clang box). The macOS branch
is unchanged.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: rkennke

build-logic/conventions/src/main/kotlin/com/datadoghq/native/config/ConfigurationPresets.kt

@@ -51,7 +51,7 @@ object ConfigurationPresets {
                 configureTsan(this, currentPlatform, currentArch, version, rootDir, compiler)
             }
             register("fuzzer") {
-                configureFuzzer(this, currentPlatform, currentArch, version, rootDir)
+                configureFuzzer(this, currentPlatform, currentArch, version, rootDir, compiler)
             }
         }
 
@@ -313,7 +313,8 @@ object ConfigurationPresets {
         platform: Platform,
         architecture: Architecture,
         version: String,
-        rootDir: File
+        rootDir: File,
+        compiler: String = "gcc"
     ) {
         config.platform.set(platform)
         config.architecture.set(architecture)
@@ -339,7 +340,28 @@ object ConfigurationPresets {
         when (platform) {
             Platform.LINUX -> {
                 config.compilerArgs.set(fuzzerCompilerArgs + commonLinuxCompilerArgs(version))
-                config.linkerArgs.set(commonLinuxLinkerArgs() + fuzzerLinkerArgs)
+
+                // commonLinuxLinkerArgs() carries -Wl,-z,defs, which forbids the
+                // undefined __asan_*/__ubsan_* symbols the instrumented objects
+                // reference. -fsanitize=address only links the runtime into
+                // executables, not shared libraries, so the fuzzer .so needs the
+                // runtime linked explicitly — same treatment (and rationale) as the
+                // asan config. On clang this resolves both __asan_* and __ubsan_*
+                // from one clang_rt.asan runtime; on gcc it adds -lubsan.
+                val libasan = PlatformUtils.locateLibasan(compiler)
+                val fuzzerRuntimeArgs = if (libasan != null) {
+                    val asanLibDir = File(libasan).parent
+                    val asanLibName = File(libasan).nameWithoutExtension.removePrefix("lib")
+                    val ubsanLibs = if (asanLibName.startsWith("clang_rt")) emptyList()
+                                    else listOf("-lubsan")
+                    listOf("-L$asanLibDir", "-l$asanLibName",
+                           "-Wl,-rpath,$asanLibDir") +
+                    ubsanLibs +
+                    fuzzerLinkerArgs
+                } else {
+                    fuzzerLinkerArgs
+                }
+                config.linkerArgs.set(commonLinuxLinkerArgs() + fuzzerRuntimeArgs)
 
                 config.testEnvironment.apply {
                     put("ASAN_OPTIONS", "allocator_may_return_null=1:detect_stack_use_after_return=0:handle_segv=0:abort_on_error=1:symbolize=1:suppressions=$rootDir/gradle/sanitizers/asan.supp")