Subzero: Fix target attribute when SZTARGET is defined.

Subzero: Fix target attribute when SZTARGET is defined.

Adds the right macro voodoo so that when Subzero is built for a single target, e.g.:

$ make -f Makefile.standalone SZTARGET=ARM32

we get output like this:

$ ./pnacl-sz --build-atts
target_ARM32

rather than this:

$ ./pnacl-sz --build-atts
target_SZTARGET

BUG= none
R=eholk@chromium.org

Review URL: https://codereview.chromium.org/2306273002 .