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* 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>