configure: don't enable $ARCH_external if $ARCH is disabled

The check_x86asm() checks would force enable these variables on success,
bypassing any --disable-* command line option.
This is important in the case of AVX512, where the relevant define is used
to choose between different values for memory alignment and strides in
some allocations.
Signed-off-by: NJames Almer <jamrial@gmail.com>

configure

@@ -5934,10 +5934,10 @@ EOF
             elf*) enabled debug && append X86ASMFLAGS $x86asm_debug ;;
         esac
 
-        check_x86asm avx512_external "vmovdqa32 [eax]{k1}{z}, zmm0"
-        check_x86asm avx2_external   "vextracti128 xmm0, ymm0, 0"
-        check_x86asm xop_external    "vpmacsdd xmm0, xmm1, xmm2, xmm3"
-        check_x86asm fma4_external   "vfmaddps ymm0, ymm1, ymm2, ymm3"
+        enabled avx512 && check_x86asm avx512_external "vmovdqa32 [eax]{k1}{z}, zmm0"
+        enabled avx2   && check_x86asm avx2_external   "vextracti128 xmm0, ymm0, 0"
+        enabled xop    && check_x86asm xop_external    "vpmacsdd xmm0, xmm1, xmm2, xmm3"
+        enabled fma4   && check_x86asm fma4_external   "vfmaddps ymm0, ymm1, ymm2, ymm3"
         check_x86asm cpunop          "CPU amdnop"
     fi