Allow the static chain to be set from C

Allow the static chain to be set from C

We need to be able to set the static chain on a few calls within the
Go runtime, so expose this with __builtin_call_with_static_chain.

        * c-family/c-common.c (c_common_reswords): Add
        __builtin_call_with_static_chain.
        * c-family/c-common.h (RID_BUILTIN_CALL_WITH_STATIC_CHAIN): New.
        * c/c-parser.c (c_parser_postfix_expression): Handle it.
        * doc/extend.texi (__builtin_call_with_static_chain): Document it.

From-SVN: r217771