EXPERIMENT: Add nongnu.org libunwind as alternative unwinding backend

NOT FOR REVIEW

Add optional support for nongnu.org libunwind as an alternative to
Android's libunwindstack for DWARF-based stack unwinding in both
traced_perf and heapprofd. Selected at build time via the GN flag
enable_perfetto_libunwind.

libunwindstack pulls in several vendored Android dependencies
(android-libprocinfo, android-libbase, bionic kernel UAPI headers)
making it difficult to use perfetto's profiling tools on non-Android
platforms. With this change, setting enable_perfetto_libunwind=true
builds everything without any libunwindstack dependency.

The refactoring introduces:

- An UnwindBackend interface with two implementations:
  LibunwindstackBackend (wrapping existing logic) and LibunwindBackend
  (using libunwind's remote API with custom accessors for sampled
  stack overlays and /proc/pid/mem).
- Backend-agnostic types (UnwindFrame, UnwindErrorCode, RegisterData)
  replacing direct use of unwindstack types throughout the pipeline.
- Perfetto-owned register capture (regs_common.h, regs_local.h, .S)
  so the heapprofd client no longer depends on libunwindstack for
  AsmGetRegs, ArchEnum, or register constants.
- Wire protocol updated to use Perfetto-owned types, maintaining ABI
  compatibility with existing register layouts.
- Bookkeeping simplified to use UnwindFrame directly, removing the
  intermediate unwindstack::FrameData conversion layer.

Also fixes nongnu libunwind's Gget_proc_info_in_range.c which
incorrectly dereferenced a remote .eh_frame_hdr address as a local
pointer, causing crashes during remote unwinding.

libunwind v1.8.0 is vendored via tools/install-build-deps and built
from source with UNW_REMOTE_ONLY (no local unwinding assembly).
Pre-baked config headers live in buildtools/nongnu_libunwind_config/.

On non-Android Linux standalone builds, enable_perfetto_libunwind
defaults to true.