[efi] Inhibit harmless ld warning on unresolved symbol check

The check for unresolved symbols does not explicitly specify an output
architecture format, and so causes a warning when building an i386 EFI
binary on an x86_64 platform.  This warning is harmless, and
specifying the output architecture in multiple places is cumbersome,
so just inhibit the warning.

src/arch/i386/Makefile.efi

 #
 $(BIN)/%.efi.tmp-reloc : $(BIN)/%.efi.tmp $(EFILINK)
 	$(QM)$(ECHO) "  [EFILINK] $@"
-	$(Q)$(LD) -e 0 -o /dev/null $< # Check for unresolved symbols
+	$(Q)# Check for unresolved symbols
+	$(Q)$(LD) -e 0 --no-warn-mismatch -o /dev/null $<
 	$(Q)$(EFILINK) $< $@
 
 $(BIN)/%.efi : $(BIN)/%.efi.tmp-reloc

src/arch/x86_64/Makefile.efi

 #
 $(BIN)/%.efi.tmp-reloc : $(BIN)/%.efi.tmp $(EFILINK)
 	$(QM)$(ECHO) "  [EFILINK] $@"
-	$(Q)$(LD) -e 0 -o /dev/null $< # Check for unresolved symbols
+	$(Q)# Check for unresolved symbols
+	$(Q)$(LD) -e 0 --no-warn-mismatch -o /dev/null $<
 	$(Q)$(EFILINK) $< $@
 
 $(BIN)/%.efi : $(BIN)/%.efi.tmp-reloc