[ARM] Error out on apple darwin platforms if float-abi is "hard".

Error out if the user tries to use float-abi="hard" since it isn't
supported on darwin platforms. Previously clang issued no warnings or
erros and just passed the option to the backend, which had no effect on
code generation for targets using apcs.

rdar://problem/22257950

Differential Revision: http://reviews.llvm.org/D12155

llvm-svn: 245866