BPF: misc minor review fixes

compiler/rustc_target/src/spec/bpf_base.rs

@@ -12,6 +12,10 @@ pub fn opts(endian: Endian) -> TargetOptions {
         no_builtins: true,
         panic_strategy: PanicStrategy::Abort,
         position_independent_executables: true,
+        // Disable MergeFunctions since:
+        // - older kernels don't support bpf-to-bpf calls
+        // - on newer kernels, userspace still needs to relocate before calling
+        //   BPF_PROG_LOAD and not all BPF libraries do that yet
         merge_functions: MergeFunctions::Disabled,
         obj_is_bitcode: true,
         requires_lto: false,