ARM: add some integer/floating point conversion libcalls

Add some Windows on ARM specific library calls.  These are provided by msvcrt,
and can be used to perform integer to floating-point conversions (and
vice-versa) mirroring similar functions in the RTABI.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208949 91177308-0d34-0410-b5e6-96231b3b80d8