ZJIT: Fix assert failure on JIT combo build

Previously, when `./configure --enable-yjit=dev --enable-zjit` JIT_DESCRIPTION was
defined to be "ZJIT", but when running with `--yjit`, ruby_set_yjit_description()
calls `define_ruby_description("+YJIT dev")`, which is longer than the
following assertion expects:

    RUBY_ASSERT(n <= ruby_description_opt_point + (int)rb_strlen_lit(JIT_DESCRIPTION));

Use the concatenation of the two strings -- the contents of the string
is never used. Minimize the scope of the macro, too.

version.c

@@ -72,11 +72,6 @@ const int ruby_api_version[] = {
 #else
 #define ZJIT_DESCRIPTION " +ZJIT"
 #endif
-#if USE_ZJIT
-#define JIT_DESCRIPTION ZJIT_DESCRIPTION
-#else
-#define JIT_DESCRIPTION YJIT_DESCRIPTION
-#endif
 #if USE_MODULAR_GC
 #define GC_DESCRIPTION " +GC"
 #else
@@ -207,6 +202,8 @@ rb_ruby_default_parser_set(ruby_default_parser_enum parser)
 static void
 define_ruby_description(const char *const jit_opt)
 {
+#define JIT_DESCRIPTION YJIT_DESCRIPTION ZJIT_DESCRIPTION
+
     static char desc[
         sizeof(ruby_description)
         + rb_strlen_lit(JIT_DESCRIPTION)
@@ -248,6 +245,7 @@ define_ruby_description(const char *const jit_opt)
      * The full ruby version string, like <tt>ruby -v</tt> prints
      */
     rb_define_const(mRuby, "DESCRIPTION", /* MKSTR(description) */ description);
+#undef JIT_DESCRIPTION
 }
 
 void