libunwind: disable building with ssp

libunwind: disable building with ssp

If we enable -fstack-protector while building libunwind, function
__stack_chk_fail_local will be referred to for i386 and powerpc32
arches.  This will cause link failure because the default gcc build
specs says no link_ssp if -nostdlib is given.

The error message:

    OpenWrt-libtool: link: ccache_cc -shared  -fPIC -DPIC  .libs/os-linux.o mi/.libs/init.o mi/.libs/flush_cache.o mi/.libs/mempool.o mi/.libs/strerror.o x86/.libs/is_fpreg.o x86/.libs/regname.o x86/.libs/Los-linux.o mi/.libs/backtrace.o mi/.libs/dyn-cancel.o mi/.libs/dyn-info-list.o mi/.libs/dyn-register.o mi/.libs/Ldyn-extract.o mi/.libs/Lfind_dynamic_proc_info.o mi/.libs/Lget_accessors.o mi/.libs/Lget_proc_info_by_ip.o mi/.libs/Lget_proc_name.o mi/.libs/Lput_dynamic_unwind_info.o mi/.libs/Ldestroy_addr_space.o mi/.libs/Lget_reg.o mi/.libs/Lset_reg.o mi/.libs/Lget_fpreg.o mi/.libs/Lset_fpreg.o mi/.libs/Lset_caching_policy.o x86/.libs/Lcreate_addr_space.o x86/.libs/Lget_save_loc.o x86/.libs/Lglobal.o x86/.libs/Linit.o x86/.libs/Linit_local.o x86/.libs/Linit_remote.o x86/.libs/Lget_proc_info.o x86/.libs/Lregs.o x86/.libs/Lresume.o x86/.libs/Lstep.o x86/.libs/getcontext-linux.o  -Wl,--whole-archive ./.libs/libunwind-dwarf-local.a ./.libs/libunwind-elf32.a -Wl,--no-whole-archive  -L/var/lib/bbmnt/buildbot/slaves/dave-builder/i386_i486/build/sdk/staging_dir/target-i386_i486_musl-1.1.16/usr/lib -L/var/lib/bbmnt/buildbot/slaves/dave-builder/i386_i486/build/sdk/staging_dir/target-i386_i486_musl-1.1.16/lib -L/var/lib/bbmnt/buildbot/slaves/dave-builder/i386_i486/build/sdk/staging_dir/toolchain-i386_i486_gcc-5.4.0_musl-1.1.16/usr/lib -L/var/lib/bbmnt/buildbot/slaves/dave-builder/i386_i486/build/sdk/staging_dir/toolchain-i386_i486_gcc-5.4.0_musl-1.1.16/lib -lc -lgcc  -Os -march=i486 -fstack-protector -Wl,-z -Wl,now -Wl,-z -Wl,relro -nostartfiles -nostdlib   -Wl,-soname -Wl,libunwind.so.8 -o .libs/libunwind.so.8.0.1
    .libs/os-linux.o: In function `_Ux86_get_elf_image':
    os-linux.c:(.text+0x588): undefined reference to `__stack_chk_fail_local'
    x86/.libs/Lregs.o: In function `_ULx86_access_fpreg':
    Lregs.c:(.text+0x25b): undefined reference to `__stack_chk_fail_local'
    x86/.libs/Lresume.o: In function `_ULx86_resume':
    Lresume.c:(.text+0xdc): undefined reference to `__stack_chk_fail_local'
    collect2: error: ld returned 1 exit status
    Makefile:2249: recipe for target 'libunwind.la' failed

The snippet from gcc -dumpspecs

    %{!nostdlib:%{!nodefaultlibs:%(link_ssp) %(link_gcc_c_sequence)}}

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>