Hide libunwind's _Unwind_* symbols under Bazel to fix bthread_tracer crash

[chenBright]

What problem does this PR solve?

Issue Number: resolve

Problem Summary:

When --define=with_bthread_tracer=true is enabled under Bazel
(bazel test //test:bthread_unittest), tests such as
BthreadTest.bthread_exit segfault inside pthread_exit /
__gxx_personality_v0, with a stack like:

#0  0x0000000000000000 in ?? ()
#1  0x00007fa2b5d6458a in _ULx86_64_dwarf_find_proc_info ()
   from /root/.cache/bazel/_bazel_root/743b333b2429a1dbd390ef66b59c771d/execroot/_main/bazel-out/k8-fastbuild/bin/test/../_solib_k8/libexternal_Slibunwind~_Slibunwind.so
#2  0x00007fa2b5d6668d in fetch_proc_info ()
   from /root/.cache/bazel/_bazel_root/743b333b2429a1dbd390ef66b59c771d/execroot/_main/bazel-out/k8-fastbuild/bin/test/../_solib_k8/libexternal_Slibunwind~_Slibunwind.so
#3  0x00007fa2b5d681a1 in _ULx86_64_dwarf_make_proc_info ()
   from /root/.cache/bazel/_bazel_root/743b333b2429a1dbd390ef66b59c771d/execroot/_main/bazel-out/k8-fastbuild/bin/test/../_solib_k8/libexternal_Slibunwind~_Slibunwind.so
#4  0x00007fa2b5d70cfd in _ULx86_64_get_proc_info ()
   from /root/.cache/bazel/_bazel_root/743b333b2429a1dbd390ef66b59c771d/execroot/_main/bazel-out/k8-fastbuild/bin/test/../_solib_k8/libexternal_Slibunwind~_Slibunwind.so
#5  0x00007fa2b5d6c775 in __libunwind_Unwind_GetLanguageSpecificData ()
   from /root/.cache/bazel/_bazel_root/743b333b2429a1dbd390ef66b59c771d/execroot/_main/bazel-out/k8-fastbuild/bin/test/../_solib_k8/libexternal_Slibunwind~_Slibunwind.so
#6  0x00007fa2b503c6df in __gxx_personality_v0 () from /lib/x86_64-linux-gnu/libstdc++.so.6
#7  0x00007fa2b5452ce5 in ?? () from /lib/x86_64-linux-gnu/libgcc_s.so.1
#8  0x00007fa2b54533c0 in _Unwind_ForcedUnwind () from /lib/x86_64-linux-gnu/libgcc_s.so.1
#9  0x00007fa2b4ca57a4 in __GI___pthread_unwind (buf=<optimized out>) at ./nptl/unwind.c:130
#10 0x00007fa2b4c9dd22 in __do_cancel () at ../sysdeps/nptl/pthreadP.h:271
#11 __GI___pthread_exit (value=0x0) at ./nptl/pthread_exit.c:36
#12 0x0000000000000000 in ?? ()

Root cause: libunwind's src/unwind/*.c exports the _Unwind_* GCC ABI
symbols, which collide with the same symbols in libgcc_s.so.1. Bazel's
default fastbuild mode wraps each cc_library into an intermediate .so,
so libexternal_S~libunwind.so ends up earlier in the binary's
DT_NEEDED than libgcc_s.so.1, and the runtime dynamic linker resolves
the _Unwind_* calls coming from pthread_exit / libstdc++ to
libunwind's DWARF-based implementation instead of libgcc_s.

This is a runtime symbol-resolution-order issue independent of the
compiler -- Clang's default toolchain on Linux uses the same
libstdc++ + libgcc_s runtime and reproduces the exact same crash.

The make / cmake CI does not hit this because it links the
autoconf-built libunwind which hides _Unwind_* by default.
Distro packages (libunwind-dev, libunwind-devel) typically
also re-export the symbols, so they have the same risk.

What is changed and the side effects?

Changed:

• New self-maintained Bazel module registry under
  registry/modules/libunwind/, hosting two variants of libunwind:
  1.8.1.brpc-no-unwind and 1.8.3.brpc-no-unwind. Both are forks of
  the corresponding modules in Bazel Central Registry, Apache-2.0 licensed.
  License attribution and a per-file change list are added at the bottom
  of the top-level LICENSE file.
• The variant overlays add a hide_unwind_symbols config_setting
  gated by --define=libunwind_hide_unwind_symbols=true. When the switch
  is on, the src/unwind/*.c glob is dropped from the build via
  select(), so libunwind no longer provides _Unwind_* symbols and
  libgcc_s.so.1 wins the runtime lookup.

Side effects:

• Performance effects:

• Breaking backward compatibility:

---

Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
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