This adds new F2018 features, that are not really enabled (because their runtime support is optional). 1. Add the new IEEE_AWAY rounding mode. It is unsupported on all known targets, but could be supported by glibc and AIX as part of the C2x proposal. Testing for now is minimal, but once a target supports that rounding mode, the tests will fail and we can fix them by expanding them. 2. Add the optional RADIX argument to IEEE_SET_ROUNDING_MODE and IEEE_GET_ROUNDING_MODE. It is unused for now, because we do not support floating-point radices other than 2. Regression-tested on x86_64-pc-linux-gnu, both 32- and 64-bit. OK to commit?