[compiler-rt] On Darwin, link all frameworks with -fapplication-extension

The ASan dylib as well as other compiler-rt dylibs work with app extensions,
so we should add -fapplication-extension to the link line when building them.
This will avoid linker warnings when using the dylibs in app extensions.

(APIs unavailable to app extensions are listed here: https://developer.apple.com/library/ios/documentation/General/Conceptual/ExtensibilityPG/ExtensionOverview.html#//apple_ref/doc/uid/TP40014214-CH2-SW6)

Differential Revision: http://reviews.llvm.org/D15550

llvm-svn: 256989