[libc] Disable the strtod and strtold tests on NVPTX

These tests have a single line that fails with a value off-by-one, see
https://lab.llvm.org/buildbot/#/builders/46/builds/50055/steps/12/logs/stdio .
Disable these for now so we can figure out what the error is later.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D153056