[OpenMP][deviceRTLs] Drop `assert` in common parts of `deviceRTLs`

The header `assert.h` needs to be included in order to use `assert` in the code.
When building NVPTX `deviceRTLs` on a CUDA free system, it requires headers from
`gcc-multilib`, which some systems don't have. This patch drops the use of
`assert` in common parts of `deviceRTLs`. In light of
`openmp/libomptarget/deviceRTLs/amdgcn/src/target_impl.h`, a code block
```
if (!cond)
  __builtin_trap();
```
is being used. The builtin will be translated to `call void @llvm.trap()`, and
the corresponding PTX is `trap;`.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D95986