[BPF] get external linkage for calling convetion functions

[inclyc]

NFC.

For support calling conv in gisel, sharing code with BPFISelLowering.

[eddyz87]

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.

[inclyc]

mark this as a draft PR because gisel code is not ready.