invoke.texi (Option Summary, [...]): Document -mserialize-volatile and -mno-serialize-volatile Xtensa options.

        * doc/invoke.texi (Option Summary, Xtensa Options): Document
        -mserialize-volatile and -mno-serialize-volatile Xtensa options.
        * config/xtensa/xtensa.c (print_operand): Do not emit MEMW instructions
        unless TARGET_SERIALIZE_VOLATILE is enabled.
        * config/xtensa/xtensa.md (*lsiu, *ssiu): Likewise.
        * config/xtensa/xtensa.h (TARGET_DEFAULT): Add MASK_SERIALIZE_VOLATILE.
        * config/xtensa/xtensa.opt (mserialize_volatile): New option.

From-SVN: r133234