oss-fuzz: link AFL++ compiler runtime for AFL builds (#296)

The OSS-Fuzz AFL build failed during linking with unresolved AFL++
symbols, including:

```
undefined symbol: __afl_manual_init
undefined symbol: __afl_fuzz_ptr
undefined symbol: __afl_persistent_loop
```

These symbols are referenced by `aflpp_driver.o` in the OSS-Fuzz fuzzing
engine archive.

When `FUZZING_ENGINE=afl`, add `/src/aflplusplus/afl-compiler-rt.o` to
Bazel's link options so the AFL++ runtime definitions are included in
the final link.

Signed-off-by: xingyaner <longfor2025@gmail.com>

fuzzing/private/oss_fuzz/repository.bzl

diff --git a/fuzzing/private/oss_fuzz/repository.bzl b/fuzzing/private/oss_fuzz/repository.bzl
index 0f81811..57b93b9 100644
--- a/fuzzing/private/oss_fuzz/repository.bzl
+++ b/fuzzing/private/oss_fuzz/repository.bzl
@@ -28,6 +28,9 @@
     instrum_conlyopts = []
     instrum_cxxopts = []
 
+    if repository_ctx.os.environ.get("FUZZING_ENGINE") == "afl":
+        stub_linkopts.append("/src/aflplusplus/afl-compiler-rt.o")
+
     if sanitizer == "undefined":
         # Bazel uses clang, not clang++, as the linker, which does not link the
         # C++ UBSan runtime library by default, but can be instructed to do so