Make the libdw dependency of C++ backtraces optional

[PerryLink]

DeepJIT is header-only, and 17 of its 27 public headers transitively include include/deep_jit/utils/exception.hpp. That file unconditionally did #include <elfutils/libdwfl.h>, so on a machine that has the libdw runtime but not the elfutils development headers, no DeepJIT header compiles — including tests/test_exception.py, the only test that does not need a GPU. Stock Ubuntu 24.04 is exactly that case: libdw1t64 is installed, libdw-dev is not, and the requirement is not mentioned anywhere in the README.

Reproduction with the same flags as validate_header_self_containment in tests/test_cuda.py (Ubuntu 24.04.4, g++ 14.4.0, no libdw-dev):

include/deep_jit/utils/exception.hpp:3:10: fatal error: elfutils/libdwfl.h: No such file or directory
    3 | #include <elfutils/libdwfl.h>
      |          ^~~~~~~~~~~~~~~~~~~~

Sweeping include/deep_jit/**/*.hpp (excluding backend/ascend/) the same way, 13 headers fail on that single line.

The library already treats libdw as optional at runtime — DwflApi's constructor does dlopen("libdw.so.1", RTLD_LAZY | RTLD_LOCAL) and valid() gates every use of the loaded symbols. Only the header was an unconditional requirement.

This makes the include conditional on __has_include and supplies a no-op DwflApi when the headers are absent, so backtraces fall back to the existing dladdr path. The same sweep after the change:

PASS=17 FAIL=5
    0 failures on elfutils/libdwfl.h
    the remaining 5 are this machine lacking cusparse.h / Python.h, unrelated

Backtraces on a header-less machine keep demangled function names and the file:line in the panic message, and report ??:0 per frame. tests/test_exception.py needs no change — it still sees panic_leaf, panic_middle, panic_entry and .cpp: in the message. On a machine with libdw-dev the guarded branch is byte-identical to before, so nothing changes there; I could not build that variant locally because installing libdw-dev requires root.