use of TOK_alloca breaks cross compiler build

VLA inserts a call to alloca via enum TOK_alloca, but TOK_alloca
only exists on I386 and X86_64 targets.  This patch just emits an
error at compile-time if someone tries to compile some VLA code
for a TOK_alloca-less target. The best solution might be to just
push the problem to link-time, since the existence-or-not of a
alloca implementation can only be determined by linking.  It seems
like just declaring TOK_alloca unconditionally would achieve that,
but for now, this at least gets the cross compilers to build.

tccgen.c

@@ -4863,6 +4863,7 @@ static void decl_initializer(CType *type, Section *sec, unsigned long c,
     CType *t1;
 
     if (type->t & VT_VLA) {
+#if defined TCC_TARGET_I386 || defined TCC_TARGET_X86_64
         int a;
         CValue retcval;
 
@@ -4877,6 +4878,9 @@ static void decl_initializer(CType *type, Section *sec, unsigned long c,
         vswap();
         vstore();
         vpop();
+#else
+        error("variable length arrays unsupported for this target");
+#endif
     } else if (type->t & VT_ARRAY) {
         s = type->ref;
         n = s->c;