* 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>
