* flags.h:  Add flag_data_sections.
        * toplev.c:  Add option -fdata-sections.  Add flag_data_sections.
        (compile_file):  Error if flag_data_sections not supported.
        * varasm.c (assemble_variable):  Handle flag_data_sections.
        * config/svr4.h:  Modify prefixes for UNIQUE_SECTION_NAME.
        * config/mips/elf.h:  Likewise.
        * config/mips/elf64.h:  Likewise.
        * invoke.texi:  Describe -fdata-sections.

From-SVN: r22619