Specify GCC flag -frounding-math on x86 (#873)

* Specify GCC flag `-frounding-math` on x86

We have been seeing fails in `test_conversions` on x86_64 when converting to the following integer types from `cl_double` with a non-default rounding mode:

```
 char_rtn_double
 char_rtp_double
 int_rtn_double
 int_rtp_double
 long_rtn_double
 long_rtp_double
 long_sat_rtn_double
 long_sat_rtp_double
 short_rtn_double
 short_rtp_double
 uchar_rtp_double
 uint_rtp_double
 ulong_rtp_double
 ushort_rtp_double
```

After investigation it was discovered that `rint()` was incorrectly rounding `cl_double` inputs despite the rounding mode being correctly set using `fesetround()` beforehand. E.g  For 'char_rtn_double' `-3.5` was getting rounding to `-3.0` rather than `-4.0`.

This is a gcc issue https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92164 when using the builtin `rint()` implementation, rather than `std::rint()`, that only presents itself when compiling a Release build with `-O2` rather than Debug.

Adding the compiler flag `-fno-builtin-rint` to the CMake works around this problem, however based on the discussion in the gcc ticket using `-frounding-math` appears to be a more comprehensive fix. As `-frounding-math` tells gcc that the code will be modifying the rounding mode, removing the assumption that the same rounding mode is in effect everywhere.

* Set FENV_ACCESS ON in rounding_mode.h

Inform compilers which are aware of the `FENV_ACCESS` pragma that TUs
which include `harness/rounding_mode.h` may manipulate the floating
point environment.

* Remove FENV_ACCESS pragma from test_conversions.cpp

This pragma is now set in the included header
`test_common/harness/rounding_mode.h`

Co-authored-by: Kenneth Benzie (Benie) <k.benzie@codeplay.com>

CMakeLists.txt

@@ -166,6 +166,8 @@ if(CMAKE_COMPILER_IS_GNUCC OR "${CMAKE_CXX_COMPILER_ID}" MATCHES "(Apple)?Clang"
             STREQUAL "x86")
         set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -msse -msse2 -mfpmath=sse")
         set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -msse -msse2 -mfpmath=sse")
+
+        add_cxx_flag_if_supported(-frounding-math)
     endif()
 else()
     set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} /D__SSE__")