Fix build with llvm-libunwind, and also fix for non-llvm libunwind
when using clang+bfd given clang doesn't have _Unwind_Find_FDE in
its rtlib and expects it from llvm-libunwind.

The _CONFIG_H_ check is for <llvm-libunwind-15 (adds _VERSION).
--- a/dlls/ntdll/unix/dwarf.h
+++ b/dlls/ntdll/unix/dwarf.h
@@ -257,5 +257,7 @@
 };
 
+#if defined(_LIBUNWIND_VERSION) || defined(____LIBUNWIND_CONFIG_H__) || !defined(__clang__)
 extern const struct dwarf_fde *_Unwind_Find_FDE (void *, struct dwarf_eh_bases *);
+#endif
 
 static unsigned char dwarf_get_u1( const unsigned char **p )
--- a/dlls/ntdll/unix/signal_x86_64.c
+++ b/dlls/ntdll/unix/signal_x86_64.c
@@ -648,5 +648,5 @@
     int rc;
 
-#ifdef __APPLE__
+#if defined(__APPLE__) || defined(_LIBUNWIND_VERSION) || defined(____LIBUNWIND_CONFIG_H__)
     rc = unw_getcontext( &unw_context );
     if (rc == UNW_ESUCCESS)
@@ -774,4 +774,5 @@
     CONTEXT *context = params->context;
     struct dwarf_eh_bases bases;
+#if defined(_LIBUNWIND_VERSION) || defined(____LIBUNWIND_CONFIG_H__) || !defined(__clang__)
     const struct dwarf_fde *fde = _Unwind_Find_FDE( (void *)(context->Rip - 1), &bases );
 
@@ -779,4 +780,5 @@
         return dwarf_virtual_unwind( context->Rip, &dispatch->EstablisherFrame, context, fde,
                                      &bases, &dispatch->LanguageHandler, &dispatch->HandlerData );
+#endif
 #ifdef HAVE_LIBUNWIND
     return libunwind_virtual_unwind( context->Rip, &dispatch->EstablisherFrame, context,