Turn off warnings for building aicasm. Various versions of yacc exist,

Turn off warnings for building aicasm.  Various versions of yacc exist,
which can result in different warnings-as-errors in this tool, but these
are all completely harmless.

This is a direct commit to stable/7, stable/8, stable/9 and stable/10,
since this kernel build tool has already been removed in head.

git-svn-id: svn://svn.freebsd.org/base/stable/10@263665 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f