Driver: support ARM/HF on a single toolchain

ARM EABI adds target attributes to the object file.  Amongst the attributes that
are emitted is the VFP argument passing (Hard vs Soft).  The linker is
responsible for checking these attributes and erroring on mismatches.  This
causes problems for the compiler-rt builtins when targeting both hard and
soft.  Because both of these options name the builtins compiler-rt component
the same (libclang_rt.builtins-arm.a or libclang_rt.builtins-arm-android).  GCC
is able to get away with this as it does one target per toolchain.  This
changes the naming convention for the ARM compiler-rt builtins to differentiate
between HF and Soft.  Although this means that compiler-rt may be duplicated, it
enables supporting both variants from a single toolchain.  A similar approach is
taken by the Darwin toolchain, naming the library to differentiate between the
calling conventions.

llvm-svn: 248649