[libunwind] Build with -funwind-tables

This adds the -funwind-tables flag when compiling libunwind. This allows
unwinding past, eg. _Unwind_RaiseException.
The upstream cmake build adds it when it is supported, and generation of
unwind tables is already done for all the libunwind C++ code since
it compiles with exceptions enabled.

Also pass -fexceptions to the C code on Fuchsia only to work around
https://bugs.llvm.org/show_bug.cgi?id=45875.

Bug: 1078588
Change-Id: Id821f143c12e7e1e376eea534be52c6cd8504465
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2218273
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774598}

buildtools/third_party/libunwind/BUILD.gn

@@ -11,7 +11,18 @@ config("libunwind_config") {
 
     # ValueAsBitPattern in Unwind-EHABI.cpp is only used on Debug builds.
     "-Wno-unused-function",
+
+    # libunwind expects to be compiled with unwind tables so it can
+    # unwind its own frames.
+    "-funwind-tables",
   ]
+
+  if (is_fuchsia) {
+    # Workaround for https://bugs.llvm.org/show_bug.cgi?id=45875:
+    # compiling the C code with -fexceptions ensures that accurate
+    # unwinding information is generated for _Unwind_RaiseException.
+    cflags_c = [ "-fexceptions" ]
+  }
 }
 
 source_set("libunwind") {