On 64-bit platforms, libgcc doesn't ship with __clzsi2, so __builtin_clz
gets lowered to __clzdi2. A check already exists for GCC, but as of
commit 8210ca019839fc5430b3a95d7caf5c829df3232a clang also lowers
__builtin_clz to __clzdi2 on sparc64.
Update the check so that building __clzdi2 with clang/sparc64 also
works.
5d0e26e571