[llvm] [BPF] get external linkage for calling convetion functions (PR #75439)

[via llvm-commits]

eddyz87 wrote:

Note for the reviewer:
the "Entry" switch for calling conversion declaration controls how tblgen would emit generated function prototypes, as a static function or as a function visible in `llvm` namespace. See `CallingConvEmitter::run()` in `CallingConvEmitter.cpp`.

The commit lgtm, it follows same pattern as other back-ends (e.g. ARM).
@inclyc, if you want to land this, should probably add yonghong-song as a reviewer.

https://github.com/llvm/llvm-project/pull/75439