ARM: set default float ABI based on triple.

Clang sets the float-abi target option manually, but no longer
annotates each function with its ABI. This can lead to confusing
mistmatch between "clang -emit-llvm | llc" and normal clang
invocations.

Besides which, gnueabihf actually *is* hard-float. Defaulting to soft
was just perverse.

llvm-svn: 197554