build: Filter undefined symbols from rtapi module version scripts

rtapi_app.h unconditionally declares EXPORT_SYMBOL for both
rtapi_app_main and rtapi_app_exit, so every module's .rtapi_export
section lists both names even when the module only implements
rtapi_app_main (e.g. hal/components/enum.c). LLD 17+ defaults to
--no-undefined-version (LLVM D135402) and then refuses to link:

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

Drop the -j .rtapi_export filter from the objdump call so the same
pass sees the full symbol table, and extend the awk script to track
both the exported names and the actually-defined symbols, emitting
only their intersection. GNU ld behaviour is unchanged (the new
global list is a strict subset of the old one) and LLD is happy.

Fixes #3191.

Author: Luca Toniolo

src/Makefile

@@ -1254,8 +1254,8 @@ modules: $(patsubst %.o,../rtlib/%.so,$(obj-m))
 ../rtlib/%.so:
 	$(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($$6,16))} END{print "local : * ; };"}' \
+	$(Q)objdump -w -t objects/$*.tmp \
+	| awk 'BEGIN{print "{ global :"} /\.rtapi_export/ && /rtapi_exported_/{e[substr($$NF,16)]=1; next} /\*UND\*/{next} {d[$$NF]=1} END{for(s in e) if(s in d) printf("%s;\n",s); print "local : * ; };"}' \
 	> objects/$*.ver
 	$(Q)$(CC) -shared -Bsymbolic -Wl,--version-script,objects/$*.ver -o $@ $^ -lm $(LDFLAGS)
 	$(Q)chmod -x $@

src/Makefile.modinc.in

@@ -120,8 +120,8 @@ $(foreach mod,$(patsubst %.o,%,$(obj-m)),\
 %.so:
 	$(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($$6,16))} END{print "local : * ; };"}' \
+	$(Q)objdump -w -t $*.tmp \
+	| awk 'BEGIN{print "{ global :"} /\.rtapi_export/ && /rtapi_exported_/{e[substr($$NF,16)]=1; next} /\*UND\*/{next} {d[$$NF]=1} END{for(s in e) if(s in d) printf("%s;\n",s); print "local : * ; };"}' \
 	> $*.ver
 	$(Q)$(CC) -shared -Bsymbolic $(LDFLAGS) -Wl,--version-script,$*.ver -o $@ $^ -lm $(EXTRA_LDFLAGS)
 	$(Q)chmod -x $@