[RISCV] Permit tail call to an externally-defined function with weak linkage

As described in D45395 `This has been modeled after ARM's tail call opt.`
ARM's abi seems to limit weak symbol.

I did not find the limitation for RISCV. (Please correct me if I am wrong)

gcc seems to use the tail-call opt: https://godbolt.org/z/bjWE68n5o

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D143137