[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.

llvm-svn: 195132