invoke.texi (fwritable-relocated-rdata): Document new Cygwin/MinGW target option.

invoke.texi (fwritable-relocated-rdata): Document new Cygwin/MinGW target option.

        * doc/invoke.texi (fwritable-relocated-rdata): Document
        new Cygwin/MinGW target option.
        * config/i386/winnt.c (i386_pe_unique_section): Ignore
        reloc if flag -fwritable-relocated-rdata is not set.
        (i386_pe_section_type_flags): Likewise.
        * config/i386/cygming.opt (fwritable-relocated-rdata):
        Add new flag variable flag_writable_rel_rdata.

From-SVN: r185027