iamcu: suppress tests which can't possibly work

With neither --32 nor --64 passed to gas, advanced features like AVX
aren't available without explicitly enabling them.

gas/testsuite/gas/i386/i386.exp

@@ -749,7 +749,9 @@ if {![istarget "i*86-*-elfiamcu"] && [gas_64_check]} then {
 run_list_test "string-bad" ""
 run_list_test "reg-bad" ""
 run_list_test "space1" "-al"
-run_list_test "xmmword" ""
+if {![istarget "i*86-*-elfiamcu"]} then {
+    run_list_test "xmmword"
+}
 # Disabled due to large memory requirement.
 # run_dump_test rept
 run_dump_test pr19498
@@ -769,6 +771,7 @@ if {[is_elf_format] || [istarget "*-*-vxworks*"]} then {
     run_dump_test "dw2-compressed-3a"
     run_dump_test "dw2-compressed-3b"
 
+    if {![istarget "i*86-*-elfiamcu"]} then {
 	run_dump_test "property-2"
 	run_dump_test "property-3"
 	run_dump_test "property-4"
@@ -786,6 +789,8 @@ if {[is_elf_format] || [istarget "*-*-vxworks*"]} then {
 	run_dump_test "property-vldmxcsr"
 	run_dump_test "property-vpcmov"
 	run_dump_test "property-vzeroall"
+    }
+
     run_dump_test "code16-2"
 
     if {![istarget "*-*-dragonfly*"]