ARM: __gnu_h2f_ieee and __gnu_f2h_ieee always use the soft-float calling convention

By default, LLVM uses the "C" calling convention for all runtime
library functions. The half-precision FP conversion functions use the
soft-float calling convention, and are needed for some targets which
use the hard-float convention by default, so must have their calling
convention explicitly set.

llvm-svn: 215348