Fixed the way we allocate executable memory on
behalf of the JIT.  We don't need it to be writable
since we are using special APIs to write into it.

<rdar://problem/13599185>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@179077 91177308-0d34-0410-b5e6-96231b3b80d8