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

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

This is the wrong solution to the problem. BPF isn't correctly configuring its set of RuntimeLibcalls. If you removed the function from the default set, you should hit the software expansion without hitting the external call.

Whether or not the libcall is available should be treated as an ABI property, preferably directly implied by the triple without a function-level control like a subtarget feature 

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