Emit segmented-stack specific code into function prologues for

Emit segmented-stack specific code into function prologues for
X86. Modify the pass added in the previous patch to call this new
code.

This new prologues generated will call a libgcc routine (__morestack)
to allocate more stack space from the heap when required

Patch by Sanjoy Das.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@138812 91177308-0d34-0410-b5e6-96231b3b80d8