libgo: Update to revision 15193:6fdc1974457c of master library.

libgo/MERGE

libgo/Makefile.am

libgo/Makefile.in

libgo/go/builtin/builtin.go

libgo/go/compress/flate/deflate.go

libgo/go/crypto/cipher/example_test.go

libgo/go/crypto/x509/x509_test.go

libgo/go/database/sql/fakedb_test.go

libgo/go/database/sql/sql.go

libgo/go/database/sql/sql_test.go

libgo/go/debug/elf/file.go

libgo/go/debug/elf/file_test.go

libgo/go/debug/elf/testdata/hello-world-core.gz

libgo/go/encoding/csv/writer.go

libgo/go/encoding/csv/writer_test.go

libgo/go/encoding/gob/decode.go

libgo/go/encoding/json/decode.go

libgo/go/exp/gotype/gotype_test.go

libgo/go/exp/locale/collate/build/trie.go

libgo/go/exp/norm/iter.go