Add __extenddftf2 and __extendsftf2 for IEEE quad precision.

Patch by: GuanHong Liu
Differential Revision: http://reviews.llvm.org/D2802


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@209783 91177308-0d34-0410-b5e6-96231b3b80d8