[SYCL][NATIVECPU] Use mock file for native cpu lib (#14515)

This fixes the failure in the nightly
(https://github.com/intel/llvm/actions/runs/9867495007/job/27247963704)
introduced by #13979 by adding a mock
file for the Native CPU utils lib, and using the `--sysroot` option so
that the driver picks it up, similarly to how it's done for e.g.
[sycl-device-lib.cpp](https://github.com/intel/llvm/blob/sycl/clang/test/Driver/sycl-device-lib.cpp).

Author: PietroGhg

clang/test/Driver/Inputs/SYCL/lib/libsycl-nativecpu_utils.bc

@@ -1,13 +1,13 @@
-//UNSUPPORTED: (system-windows && !native_cpu)
-//RUN: %clang -fsycl -fsycl-targets=native_cpu -fsycl-libspirv-path=%S/Inputs/SYCL/libspirv.bc -ccc-print-phases %s 2>&1 | FileCheck %s --check-prefix=CHECK_ACTIONS
-//RUN: %clang -fsycl -fsycl-targets=native_cpu -fsycl-libspirv-path=%S/Inputs/SYCL/libspirv.bc -ccc-print-bindings %s 2>&1 | FileCheck %s --check-prefix=CHECK_BINDINGS
-//RUN: %clang -fsycl -fsycl-targets=native_cpu -fsycl-libspirv-path=%S/Inputs/SYCL/libspirv.bc -### %s 2>&1 | FileCheck %s --check-prefix=CHECK_INVO
-//RUN: %clang -fsycl -fsycl-targets=native_cpu -fsycl-libspirv-path=%S/Inputs/SYCL/libspirv.bc -target aarch64-unknown-linux-gnu -ccc-print-phases %s 2>&1 | FileCheck %s --check-prefix=CHECK_ACTIONS-AARCH64
+//UNSUPPORTED: system-windows
+//RUN: %clang -fsycl --sysroot=%S/Inputs/SYCL -fsycl-targets=native_cpu -fsycl-libspirv-path=%S/Inputs/SYCL/libspirv.bc -ccc-print-phases %s 2>&1 | FileCheck %s --check-prefix=CHECK_ACTIONS
+//RUN: %clang -fsycl --sysroot=%S/Inputs/SYCL -fsycl-targets=native_cpu -fsycl-libspirv-path=%S/Inputs/SYCL/libspirv.bc -ccc-print-bindings %s 2>&1 | FileCheck %s --check-prefix=CHECK_BINDINGS
+//RUN: %clang -fsycl --sysroot=%S/Inputs/SYCL -fsycl-targets=native_cpu -fsycl-libspirv-path=%S/Inputs/SYCL/libspirv.bc -### %s 2>&1 | FileCheck %s --check-prefix=CHECK_INVO
+//RUN: %clang -fsycl --sysroot=%S/Inputs/SYCL -fsycl-targets=native_cpu -fsycl-libspirv-path=%S/Inputs/SYCL/libspirv.bc -target aarch64-unknown-linux-gnu -ccc-print-phases %s 2>&1 | FileCheck %s --check-prefix=CHECK_ACTIONS-AARCH64
 
 //Link together multiple TUs.
 //RUN: touch %t_1.o 
 //RUN: touch %t_2.o 
-//RUN: %clang -fsycl -fsycl-targets=native_cpu -fsycl-libspirv-path=%S/Inputs/SYCL/libspirv.bc %t_1.o %t_2.o -ccc-print-bindings 2>&1 | FileCheck %s --check-prefix=CHECK_BINDINGS_MULTI_TU
+//RUN: %clang -fsycl -fsycl-targets=native_cpu --sysroot=%S/Inputs/SYCL -fsycl-libspirv-path=%S/Inputs/SYCL/libspirv.bc %t_1.o %t_2.o -ccc-print-bindings 2>&1 | FileCheck %s --check-prefix=CHECK_BINDINGS_MULTI_TU
 
 
 //CHECK_ACTIONS:                     +- 0: input, "{{.*}}sycl-native-cpu-fsycl.cpp", c++, (host-sycl)