build: Filter rtapi module version scripts to defined symbols only

rtapi_app.h unconditionally declares EXPORT_SYMBOL(rtapi_app_main) and
EXPORT_SYMBOL(rtapi_app_exit), causing every module to list both names
in its generated linker version script. Modules that only implement
rtapi_app_main (e.g. hal/components/enum.c) then triggered lld's
--no-undefined-version (default since LLD 17, see LLVM D135402):

    ld.lld: error: version script assignment of 'global' to symbol
    'rtapi_app_exit' failed: symbol not defined

Intersect the exported-name list with the actual defined symbols from
nm -g --defined-only so the version script only mentions symbols that
are really present in the linked object. Behavior under GNU ld is
unchanged (the new list is a strict subset of the previous one), and
LLD now accepts the build without needing --undefined-version.

Fixes #3191.

Author: Luca Toniolo

src/Makefile

@@ -1255,8 +1255,9 @@ modules: $(patsubst %.o,../rtlib/%.so,$(obj-m))
 	$(ECHO) Linking $@
 	$(Q)ld -d -r -o objects/$*.tmp $^
 	$(Q)objdump -w -j .rtapi_export -t objects/$*.tmp \
-	| awk 'BEGIN{print "{ global :"} /rtapi_exported_/{printf("%s;\n", substr($$6,16))} END{print "local : * ; };"}' \
-	> objects/$*.ver
+	| awk '/rtapi_exported_/{print substr($$NF,16)}' | sort -u > objects/$*.exported
+	$(Q)nm -g --defined-only objects/$*.tmp | awk '{print $$NF}' | sort -u > objects/$*.defsyms
+	$(Q)(echo "{ global :"; comm -12 objects/$*.exported objects/$*.defsyms | sed 's/$$/;/'; echo "local : * ; };") > objects/$*.ver
 	$(Q)$(CC) -shared -Bsymbolic -Wl,--version-script,objects/$*.ver -o $@ $^ -lm $(LDFLAGS)
 	$(Q)chmod -x $@
 

src/Makefile.modinc.in

@@ -121,8 +121,9 @@ $(foreach mod,$(patsubst %.o,%,$(obj-m)),\
 	$(ECHO) Linking $@
 	$(Q)ld -d -r -o $*.tmp $^
 	$(Q)objdump -w -j .rtapi_export -t $*.tmp \
-	| awk 'BEGIN{print "{ global :"} /rtapi_exported_/{printf("%s;\n", substr($$6,16))} END{print "local : * ; };"}' \
-	> $*.ver
+	| awk '/rtapi_exported_/{print substr($$NF,16)}' | sort -u > $*.exported
+	$(Q)nm -g --defined-only $*.tmp | awk '{print $$NF}' | sort -u > $*.defsyms
+	$(Q)(echo "{ global :"; comm -12 $*.exported $*.defsyms | sed 's/$$/;/'; echo "local : * ; };") > $*.ver
 	$(Q)$(CC) -shared -Bsymbolic $(LDFLAGS) -Wl,--version-script,$*.ver -o $@ $^ -lm $(EXTRA_LDFLAGS)
 	$(Q)chmod -x $@
 endif