[OpenMP][Offloading] Add a CMake argument LIBOMPTARGET_LIT_ARGS to control behavior...

[OpenMP][Offloading] Add a CMake argument LIBOMPTARGET_LIT_ARGS to control behavior of libomptarget lit test

By default, `lit` uses all threads to invoke tests, which  can easily cause out
of memory on GPUs because most of OpenMP offloading test usually take about 1GB
GPU memory, but a typical GPU only has 4-8GB memory. This patch introduce a
CMake argument `LIBOMPTARGET_LIT_ARGS` to allow users to control the behavior of
`libomptarget` tests, similar to `LLVM_LIT_ARGS`.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D106236