[SYCL][TEST] Skip free_function_host_compiler.cpp when using libcxx (#19696)

GNU `g++` host compiler doesn't work with libcxx without some voodoo
magic.

Author: aelovikov-intel

sycl/test/extensions/free_function_kernels/free_function_host_compiler.cpp

@@ -2,6 +2,7 @@
 // RUN: %clangxx -fsycl -fno-sycl-unnamed-lambda %s
 // RUN: %clangxx -fsycl -fsycl-host-compiler=g++ %s
 // REQUIRES: linux
+// UNSUPPORTED: libcxx
 
 // This test serves as a sanity check that compilation succeeds
 // for a simple free function kernel when using the host compiler