Make the __morestack function available to the JIT memory manager under Linux.

This function's implementation lives in libgcc, a static library, so we need
to expose it explicitly, like the other such functions.

Differential Revision: http://reviews.llvm.org/D6788

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224993 91177308-0d34-0410-b5e6-96231b3b80d8