[RuntimeDyld] Don't allocate unnecessary stub buffer space

[RuntimeDyld] Don't allocate unnecessary stub buffer space

Summary:
For relocation types that are known to not require stub functions, there
is no need to allocate extra space for the stub functions.

Reviewers: lhames, reames, maksfb

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14676

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253920 91177308-0d34-0410-b5e6-96231b3b80d8