Note: Once this CL has landed, Subzero's "make -f Makeefile.standalone
check-spec" works in the integrated assembler without using hybrid
assembly.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4334
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/1649053002 .