[builtins] Fix c?zdi2 on sparc64/Linux and ignore riscv32

On sparc64/Linux, sparc64 isn't defined; the canonical way of
checking for sparc64 is sparc && arch64, which also works on the
BSDs and Solaris. Since this problem does not occur on 32-bit
architectures, riscv32 can be ignored. This fixes and refines rL324593.

Patch by jrtc27 (James Clarke)

Differential Revision: https://reviews.llvm.org/D43146

llvm-svn: 342504