AddressSanitizer: Abort after failed exec() and get rid of the allow_reexec ASan flag

As mentioned in
https://code.google.com/p/address-sanitizer/issues/detail?id=365, when the
re-exec that adds the required DYLD_INSERT_LIBRARIES variable fails, ASan
currently continues to run, but things are broken (some memory can be
overwritten, interceptors don't work, ...). This patch aborts if the execv()
fails and prints an error message that DYLD_INSERT_LIBRARIES is required. It
also removes the "alllow_reexec" flag, since using it causes the same issues.

Reviewed at http://reviews.llvm.org/D6752