RUBY3_HAS_BUILTIN: fix __builtin_unreachable

This macro has to be truthy, otherwise the `+0` trick above evalues
RUBY3_HAS_BUILTIN(__builtin_unreachable) to be always false.

include/ruby/3/has/builtin.h

@@ -86,6 +86,6 @@
 # define RUBY3_HAS_BUILTIN___builtin_popcountll        HAVE_BUILTIN___BUILTIN_POPCOUNTLL
 # define RUBY3_HAS_BUILTIN___builtin_sub_overflow      HAVE_BUILTIN___BUILTIN_SUB_OVERFLOW
 # if defined(UNREACHABLE)
-#  define RUBY3_HAS_BUILTIN___builtin_unreachable
+#  define RUBY3_HAS_BUILTIN___builtin_unreachable 1
 # endif
 #endif