[llvm] [BPF] Allow libcalls behind a feature gate (PR #168442)

[via llvm-commits]

================
@@ -163,6 +165,14 @@ class BPFTargetLowering : public TargetLowering {
   MachineBasicBlock *
   EmitInstrWithCustomInserterLDimm64(MachineInstr &MI,
                                      MachineBasicBlock *BB) const;
+
+  /// Returns true if arguments should be sign-extended in lib calls.
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yonghong-song wrote:

Let us do
```
// Returns true if arguments should be sign-extended in lib calls.
```
similar to the comments in previous lines.

https://github.com/llvm/llvm-project/pull/168442