Fix PR35739: chkstk and chkst2 should only be built for Windows

As reported in PR35739, rL252927 added the Windows specific chkstk and
chkstk2 sources unconditionally, and since these are assembly without a
NO_EXEC_STACK_DIRECTIVE at the end, automated vulnerability scanners
warned about the objects having an executable stack.

Avoid the problem by only including these files when Windows is
targeted.

Reviewers: compnerd, rnk, martell

Reviewed By: martell

Subscribers: mstorsjo, mgorny, martell, javed.absar, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D41567

llvm-svn: 321431