Update prebuilts to go 1.12.1

From https://ci.android.com/builds/submitted/5389456/linux/latest/go.zip

Also includes a cherry-pick of
https://github.com/golang/go/commit/ff048033e4304898245d843e79ed1a0897006c6d

Fixes: 126298064
Test: m blueprint_tools
Change-Id: I581e084f909acc0b1e3f95be6452c86c86da1bac

src/math/big/prime.go

diff --git a/src/math/big/prime.go b/src/math/big/prime.go
index 4c2c152..d9a5f1e 100644
--- a/src/math/big/prime.go
+++ b/src/math/big/prime.go
@@ -51,7 +51,7 @@
 	}
 
 	if w&1 == 0 {
-		return false // n is even
+		return false // x is even
 	}
 
 	const primesA = 3 * 5 * 7 * 11 * 13 * 17 * 19 * 23 * 37