Escape genrule commands

Bug: 70387174
Test: Put this text into an Android.bp:

    genrule {
        name: "test_genrule",
        tool_files: ["foo"],
        out: ["foo.c"],
        cmd: "for i in a b c; do echo $$i; done > $(out)",
    }

    cc_library {
        name: "libtest_genrule",
        srcs: [":test_genrule"],
    }

  and then run `m -j libtest_genrule`. Although the library
  shouldn't compile, check that it produces a foo.c that has
  "a\nb\n\c\n" and not "\n\n\n".

Change-Id: I139106479439c9b3a95f1a2ecc23e73570d7bd59