upgrade the cross-gdb to 6.8 (patch from #4701)

toolchain/gdb/Makefile

toolchain/gdb/patches/100-uclibc-conf.patch

toolchain/gdb/patches/200-uclibc-readline-conf.patch

toolchain/gdb/patches/400-mips-coredump.patch

toolchain/gdb/patches/500-thread-timeout.patch

toolchain/gdb/patches/600-debian_10.selected-frame.patch

toolchain/gdb/patches/600-fix-compile-flag-mismatch.patch

toolchain/gdb/patches/620-debian_static-thread-db.patch

toolchain/gdb/patches/630-debian_24.tracepoint-segv.patch

...gdb/patches/640-debian_dwarf2-frame-signal-unwinder.patch

toolchain/gdb/patches/650-debian_vsyscall-gdb-support.patch

toolchain/gdb/patches/660-debian_dwarf-cfa-restore.patch

toolchain/gdb/patches/680-debian_sim-destdir.patch

toolchain/gdb/patches/690-debian_member-field-symtab.patch

toolchain/gdb/patches/700-debian_cp-pass-by-reference.patch

...gdb/patches/710-debian_thread-db-multiple-libraries.patch

toolchain/gdb/patches/720-debian_static-threads-test.patch

...hain/gdb/patches/730-debian_gdb-fix-tracefork-check.patch

toolchain/gdb/patches/740-debian_make-cv-type-crash.patch

toolchain/gdb/patches/750-debian_sparc-singlestep.patch