Workaround vpush/vpop instructions generation for non-FPU ARMv5 target

Issue #696 (bdwgc).

When targeting armv5te architecture, clang-19, at least, emits vpush
and vpop instructions for `__builtin_unwind_init()` regardless of
compiler flags like `-mfloat-abi=soft`.  As the target architecture
lacks FPU, the generated code crashes.
The patch prevents `HAVE_BUILTIN_UNWIND_INIT` macro definition for the
case.

* include/private/gcconfig.h [GC_GNUC_PREREQ(2,8) && __clang__
&& __ARM_ARCH_5TE__] (HAVE_BUILTIN_UNWIND_INIT): Do not define; add
comment.

include/private/gcconfig.h

@@ -796,6 +796,8 @@ EXTERN_C_BEGIN
     && !(defined(POWERPC) && defined(DARWIN)) /* for MacOS X 10.3.9 */    \
     && !defined(E2K) && !defined(RTEMS)                                   \
     && !defined(__ARMCC_VERSION) /* does not exist in armcc gnu emu */    \
+    && !(defined(__clang__)                                               \
+         && defined(__ARM_ARCH_5TE__) /* clang-19 emits vpush/vpop */)    \
     && (!defined(__clang__)                                               \
         || GC_CLANG_PREREQ(8, 0) /* was no-op in clang-3 at least */)
 #  define HAVE_BUILTIN_UNWIND_INIT