runtime: SPARCv9 fixes for lfstack.

From Rainer Orth.

R=iant
CC=gofrontend-dev
https://codereview.appspot.com/7227049
libgo: Update Go library to master revision 15489/921e53d4863c.

R=iant
CC=gofrontend-dev
https://codereview.appspot.com/7235059