[clang] [SystemZ][zOS] disable _Float16 support on z/OS (PR #145532)

[Sean Perry via cfe-commits]

perry-ca wrote:

> I guess this is fine with me if this is what you want for z/OS. Note that on Linux, _Float16 also wasn't supported previously, we've made the choice to add it.

We don't see any reason not to support this either.  However, we need to first update the z/OS ABI to ensure no binary compat issues.  We'll enable it once we synchronize everything.

https://github.com/llvm/llvm-project/pull/145532