FPGA: Set fp model to "precise" for ac_fixed sample (#1483)

* set fp-model to precise for emu only

* set fp-model to precise for all platforms

Author: wangdi4

DirectProgramming/C++SYCL_FPGA/Tutorials/Features/ac_fixed/README.md

@@ -132,6 +132,8 @@ When you use the `ac_fixed` library, keep the following points in mind:
 
      The host program for this tutorial shows the accuracy differences between the result provided by floating point math library and the result provided by the `ac_fixed` math library functions, where the `float` version generates a more accurate result than the smaller-sized `ac_fixed` version.
 
+     Note: the program is compiled with fp-model set to "precise", so the accuracy of the floating-point math functions conform to the IEEE standard.
+
   - Emulation vs FPGA Hardware for fixed point math operations
 
      Due to the differences in the internal math implementations, the results from `ac_fixed` math functions in emulation and FPGA hardware might not always be bit-accurate. This tutorial shows how to build and run the sample for emulation and FPGA hardware so you can observe the difference.

DirectProgramming/C++SYCL_FPGA/Tutorials/Features/ac_fixed/src/CMakeLists.txt

@@ -19,9 +19,9 @@ endif()
 # 2. /Qactypes Include ac_types headers and link against ac_types emulation libraries
 if(WIN32)
     set(WIN_FLAG "/EHsc")
-    set(AC_TYPES_FLAG "/Qactypes")
+    set(AC_TYPES_FLAG "/Qactypes /fp:precise")
 else()
-    set(AC_TYPES_FLAG "-qactypes")
+    set(AC_TYPES_FLAG "-qactypes -fp-model=precise")
 endif()
 
 # A SYCL ahead-of-time (AoT) compile processes the device code in two stages.