libgo: drop Solaris 10 support

Based on patch by Rainer Orth.

Change-Id: I2bd5b6e1b033857eeecbea90dfc52ec949740cfa
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/176938
Reviewed-by: Ian Lance Taylor <iant@golang.org>
4 files changed