[libc] Disable 'DecodeInOtherBases` test on GPU targets

This test is excessively slow on GPU targets, taking anywhere beween 5
and 60 seconds to complete each time it's run. See
https://lab.llvm.org/buildbot/#/builders/55/builds/52203/steps/12/logs/stdio
for an example on the NVPTX buildbot. Simply disable testing this on the
GPU for now.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D155979