Make TARGET_STATIC_CHAIN allow a function type

Make TARGET_STATIC_CHAIN allow a function type

As opposed to always being a decl.  This is a prerequisite
to allowing the static chain to be loaded for indirect calls.

        * targhooks.c (default_static_chain): Remove check for
        DECL_STATIC_CHAIN.
        * config/moxie/moxie.c (moxie_static_chain): Likewise.
        * config/i386/i386.c (ix86_static_chain): Allow decl or type
        as the first argument.
        * config/xtensa/xtensa.c (xtensa_static_chain): Change the name
        of the unused first parameter.
        * doc/tm.texi (TARGET_STATIC_CHAIN): Document the first parameter
        may be a type.
        * target.def (static_chain): Likewise.

From-SVN: r217769