[ASan] Fix unset-insert-libraries-on-exec.cc on 32-bit Darwin.
The test used to execute a 64-bit /bin/bash binary and preload a 32-bit dynamic library into it.
Now the arch-specific version of echo-env is executed instead.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@183662 91177308-0d34-0410-b5e6-96231b3b80d8