[ASan] Use -isysroot instead of -Wl,-syslibroot to link the ASan iossim runtime
(-Wl,-syslibroot was accidentally overridden by -isysroot from Clang on OSX 10.9)

-isysroot is a Clang/LLVM-GCC-specific option, but hosting libsanitizer for LLVM
with GCC on Darwin shouldn't work anyway, because of the missing blocks support.



git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@195132 91177308-0d34-0410-b5e6-96231b3b80d8