[libc] Do not run tests on vendor implemented math

We currently remap vendor implementations of math functions to provide a
temporarily functional `libm.a` for the GPU. However, we should not run
tests on any files that depend on these vendor implementations as they
are not under our control and are not always present.

The goal in the future is to remove the need for this by replacing all
the vendor functionality, but for now this is a workaround.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D158213