build: fix assembler instruction tests again

Commit 7fb252bd41 ("build/xen: fix assembler instruction tests") added
$(AFLAGS) here, which results in all of those tests now failing.
Certain items need to be removed for things to work again.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/Config.mk
+++ b/Config.mk
@@ -150,7 +150,8 @@ endif
 # as-insn: Check whether assembler supports an instruction.
 # Usage: cflags-y += $(call as-insn "insn",option-yes,option-no)
 as-insn = $(if $(shell echo 'void _(void) { asm volatile ( $(2) ); }' \
-                       | $(1) $(AFLAGS) -c -x c -o /dev/null - 2>&1),$(4),$(3))
+                       | $(1) $(filter-out -M% %.d -include %/include/xen/config.h,$(AFLAGS)) \
+                              -c -x c -o /dev/null - 2>&1),$(4),$(3))
 
 # as-insn-check: Add an option to compilation flags, but only if insn is
 #                supported by assembler.