[libc] disable flaky nanl test on riscv

[nickdesaulniers]

This test on the bot is flaky because the bot isn't fast enough and winds up timing out.

https://lab.llvm.org/buildbot/#/builders/257

cc @lntue

perhaps there's something else we can do like extend the default timeout?

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

This test on the bot is flaky because the bot isn't fast enough and winds up timing out.

https://lab.llvm.org/buildbot/#/builders/257

cc @lntue

perhaps there's something else we can do like extend the default timeout?