ARM: teach AAPCS-VFP to deal with Cortex-M4.

Cortex-M4 only has single-precision floating point support, so any LLVM
"double" type will have been split into 2 i32s by now. Fortunately, the
consecutive-register framework turns out to be precisely what's needed to
reconstruct the double and follow AAPCS-VFP correctly!

rdar://problem/17012966

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209650 91177308-0d34-0410-b5e6-96231b3b80d8