[OpenMP][CMake] Fix an issue when there is space in the argument LIBOMPTARGET_LIT_ARGS

D106236 added a new CMake argument for `libomptarget` test, but when user's
input contains white spaces, CMake will add escape char to the final lit command,
which leads to an error. This patch converts the user's input `LIBOMPTARGET_LIT_ARGS`
into a local array, and then passes the array to the function.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D106247