[Sparc] Don't claim JIT support on SPARC for now

Until D118450 <https://reviews.llvm.org/D118450> lands, there's no JIT
support on SPARC, but the backend claims otherwise, leading to various
testsuite failures.

This patch corrects this.

Tested on `sparcv9-sun-solaris2.11`.

Differential Revision: https://reviews.llvm.org/D129349