builtins: support building ARM builtins for a HF target

The `-target` impacts the CC for the builtins.  HF targets (with either
floating point ABI) always use AAPCS VFP for the builtins unless they
are AEABI builtins, in which case they use AAPCS.  Non-HF targets (with
either floating point ABI) always use AAPCS for the builtins and AAPCS
for the AEABI builtins.  This introduces the thunks necessary to switch
CC for the floating point operations.  This is not currently enabled,
and should be dependent on the target being used to build compiler-rt.
However, as a stop-gap, a define can be added for ASFLAGS to get the
thunks.

llvm-svn: 291677