ARM: match GCC's behaviour for builtins

GCC changes the CC between the user-code and the builtins based on the
value of `-target` rather than `-mfloat-abi`.  When a HF target is used,
the VFP variant of the AAPCS CC is used.  Otherwise, the AAPCS variant
is used.  In all cases, the AEABI functions use the AAPCS CC.  Adjust
the calling convention based on the target.

Resolves PR30543!

llvm-svn: 291909