[SYCL] Rework fix for -fPIC on Windows (#729)

Previous fix removes substitution causing failure below:
clang++: error: no such file or directory: '%fPIC'; did you mean '-fPIC'?

The current fix provides substitution with empty string.

Author: vladimirlaz

SYCL/lit.cfg.py

@@ -159,8 +159,7 @@
     # time providing this option for compilation targeting 
     # x86_64-pc-windows-msvc will cause compile time error on some
     # configurations
-    if platform.system() != "Windows":
-        config.substitutions.append( ('%fPIC', '-fPIC') )
+    config.substitutions.append( ('%fPIC', ('' if platform.system() == 'Windows' else '-fPIC')) )
     config.substitutions.append( ('%shared_lib', '-shared') )
 
 if not config.gpu_aot_target_opts: