Add an optional patch to fix missing symbol link error

Similar in nature to rust-lang/rust#105967
I was getting the following at the final chrome link stage :-

ld.lld: error: version script assignment of 'global' to symbol \
'__libc_csu_fini' failed: symbol not defined
ld.lld: error: version script assignment of 'global' to symbol \
'__libc_csu_init' failed: symbol not defined

The explanation at https://reviews.llvm.org/D140591#4014995 seems to
point to my self-compiled llvm/clang perhaps lacking one or both of
libcxx and libcxxabi (-DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi"),
which may have avoided the issue.

Bundled clang users probably won't need to enable the patch.

debian/patches/optional/fix-missing-symbols.patch

@@ -0,0 +1,17 @@
+--- a/build/config/compiler/BUILD.gn
++++ b/build/config/compiler/BUILD.gn
+@@ -520,6 +520,14 @@
+     }
+   }
+
++  if (use_lld) {
++    # Workaround for missing symbol link errors similar in form to the one at
++    # https://github.com/llvm/llvm-project/issues/59660 and described at
++    # https://reviews.llvm.org/D140591#4014995.
++    ldflags += [ "-Wl,--undefined-version" ]
++  }
++
++
+   # Enable text section splitting only on linux when using lld for now. Other
+   # platforms can be added later if needed.
+   if ((is_linux || is_chromeos) && use_lld && use_text_section_splitting) {

debian/patches/series.debian

@@ -131,3 +131,4 @@ various/vanadium/disable-article-suggestions-feature-by-default.patch
 #optional/system/unstable/roll-src-third_party-ffmpeg-102.patch
 #optional/system/unstable/roll-src-third_party-ffmpeg-106.patch
 #optional/system/unstable/v8-enhance-Date-parser-to-take-Unicode-SPACE.patch
+#optional/fix-missing-symbols.patch