Enable stack trace symbolization for OSS builds

Summary:
Historically, HHVM statically linked the system libbfd to power stack trace symbolization in its crash reports and in perf map files. This was disabled for OSS in D2137703[1] because of the wildly differing libbfd ABI across target systems. D3742004[2] and D3855027[3] then brought in folly::Symbolizer for both use cases, but this remained gated behind a Meta-only define.

folly::Symbolizer has been stable in upstream folly for a while now, so let's enable it unconditionally and deshim the experimental/ header. This also allows removing the dead libbfd integration. For this to work, the folly build must be built with and able to find libunwind, which isn't a given when building against libc++ and LLVM libunwind on Ubuntu as it installs includes into a subdirectory, so provide an additional hint for this case.

This allows us to finally have a proper stacktrace rather than raw memory addresses in crash reports, such as:
```
Core dumped: Segmentation fault
Stack trace in /tmp/stacktrace.902095.log
Host: hhvm-jammy-vm
ProcessID: 902095
ThreadID: 281473658339392
ThreadPID: 902095
Name: /usr/local/bin/hhvm
CmdLine: hhvm ../hhvm/hphp/test/quick/dv.php
Type: Segmentation fault
Runtime: hhvm
Version: heads/7.70.0-slack-0-g7be53eff8d22faa0383a128696fae83b224aedd5
DebuggerCount: 0

Arguments: ../hhvm/hphp/test/quick/dv.php
ThreadType: CLI

-------------------------------Treadmill Information----------------------------
Now: 235093382994348
OldestStartTime: 235093373066177
InflightRequestsSize: 1

Active Requests:
  281473658339392 1 235093373066177 (age 9ms) (timeout 0s) CLISession OLDEST

------------------------------------------------------------------------------

CPP Stacktrace:

 # 0  HPHP::jit::FixupMap::processFixupForVMFrame(HPHP::jit::VMFrame)
 # 1  HPHP::jit::FixupMap::fixupWork(HPHP::ActRec*, bool)
 # 2  HPHP::jit::detail::syncVMRegsWork(bool)
 # 3  HPHP::createBacktrace(HPHP::BacktraceArgs const&)
 # 4  HPHP::f_debug_backtrace(long, long)
 # 5  HPHP::throwable_init(HPHP::ObjectData*)
 # 6  HPHP::ObjectData* HPHP::ObjectData::newInstance<false>(HPHP::Class*)
 # 7  HPHP::SystemLib::AllocRuntimeExceptionObject(HPHP::Variant const&)
 # 8  HPHP::SystemLib::throwRuntimeExceptionObject(HPHP::Variant const&)
 # 9  HPHP::throwMissingArgument(HPHP::Func const*, int)

PHP Stacktrace:

 #0  main() called at [/home/mszabo/hhvm/hphp/test/quick/dv.php:28]
 facebook/hhvm#1  main_entry()

```

[1] facebook/hhvm@a66dd13
[2] facebook/hhvm@c11ad4e
[3] facebook/hhvm@0e082c4

X-link: facebook/hhvm#9687

Reviewed By: paulbiss

Differential Revision: D88499314

fbshipit-source-id: dc2cbd5cdeab1533ae5fee26b3bf64fc7595cd8c

Author: mszabo-wikia

build/fbcode_builder/CMake/FindLibUnwind.cmake

@@ -12,7 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-find_path(LIBUNWIND_INCLUDE_DIR NAMES libunwind.h)
+# When using prepackaged LLVM libunwind on Ubuntu, its includes are installed in a subdirectory.
+find_path(LIBUNWIND_INCLUDE_DIR NAMES libunwind.h PATH_SUFFIXES libunwind)
 mark_as_advanced(LIBUNWIND_INCLUDE_DIR)
 
 find_library(LIBUNWIND_LIBRARY NAMES unwind)