Make __gnu_h2f_ieee and __gnu_f2h_ieee visible so that hipTestHalf
test can succeed in Clang compiler + Hcc RT.
Change-Id: I5f7d5db19e559b3b66356f0170a8dbc1e5505f3e
The implementation is copied from HCC runtime.
For hcc it has no effect since apps can find them in either hcc runtime or HIP
runtime.
hip-clang needs it in HIP/HCC runtime so that HIP/HCC and HIP/VDI runtime are
swappable.
Tao Sang
Yaxun Sam Liu