Fixup omitting frame pointers on various compilers and architectures

[hughbe]

Commit 1:

• Add momit-leaf-frame-pointer on i686 architectures
• There is a comment "Add -momit-leaf-frame-pointer on x86.", so we should do this on x86

Commit 2:

• Add /Oy with MSVC/Clang-cl to omit frame pointers
• https://msdn.microsoft.com/en-us/library/2kxx5t2c.aspx

Commit 3:

• There is a comment "Add -momit-leaf-frame-pointer on x86.", so we should do this on x86_64

[hughbe]

@compnerd PTAL. The second commit I know is correct.
However, the 1st and 3rd commits rely on the comment that we omit frame pointers on x86. It currently seems like this is not the case.
So, either this is the correct fix, or we need to update the comment to say "on x86 and x86_64"

[hughbe]

@swift-ci please clean smoke test

[compnerd]

The change is correct, but please collapse the patches into a single one before pushing this.

[hughbe]

Squash and merge it is. Was unsure what the right thing to do was, so split the commits up :)