[libc] Fix GPU tests failing after recent changes

Summray:
We landed some extra math support, which is apparently broken on the
max / min functions. the `mod` functions cannot be tested as they use
`std::limits` which don't exist in a freestanding environment. Also the
`blockstore` test seems to be broken. We will need to fix these in the
future but for now we need something in a workable state.

Reviewed By: jplehr

Differential Revision: https://reviews.llvm.org/D156329