libgo: additional type/const references in sysinfo.c

Add a few more explicit references to enumeration constants
(RUSAGE_SELF, DT_UNKNOWN) in sysinfo.c to insure that their hosting enums
are emitted into DWARF, when using a clang host compiler during
the gollvm build.

Updates golang/go#41382.
Updates golang/go#41404.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/254941

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-b75a139fcc7c56988ce2d5b3a2b9e274eb521b0d
+a47485cd0e9ce6a8b3e88e53ccc0a440f0bd4351
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/sysinfo.c

@@ -316,6 +316,7 @@ enum {
 SREF(dirent);
 SREF(dirent64);
 OTREF(DIR);
+EREF(DT_UNKNOWN);
 
 // From fcntl.h
 SREF(flock);
@@ -437,6 +438,7 @@ SREF(rusage);
 SREF(rlimit64);
 EREF(RLIMIT_NOFILE);
 EREF(PRIO_USER);
+EREF(RUSAGE_SELF);
 
 // From sys/select.h
 TREF(fd_set);