i386.c (ix86_option_override_internal): Allow -mabi for 32-bit, too.

        * config/i386/i386.c (ix86_option_override_internal): Allow -mabi
        for 32-bit, too.
        (ix86_handle_abi_attribute): Allow function attributes
        ms_abi/sysv_abi in 32-bit mode, too.
        * doc/extend.texi: Adjust attribute documentation.

        * gcc.target/i386/aggregate-ret3.c: New test.
        * gcc.target/i386/aggregate-ret4.c: New test.

From-SVN: r176828