[ARM] Error out if float-ab=hard and abi=apcs-gnu on macho platforms.

Error out if -mfloat-abi=hard or -mhard-float is specified on the command
line and the target ABI is APCS. Previously clang issued no warnings or
errors and just passed the option to the backend, which had no effect on
code generation for targets using APCS.

This commit corrects the patch commited in r245866, which didn't take into
account the fact that not all darwin targets use APCS.

rdar://problem/22257950

http://reviews.llvm.org/D12344

llvm-svn: 246054