CONTRIBUTORS: first round of automated updates for Go 1.12

These updates have been automatically generated using a modified
version of the updateac command with the following changes:

-	code to automatically update the AUTHORS file has been removed,
	since we're not updating that file automatically after Go 1.11
	(see CL 128877)
-	CLA checking code has been removed, since it was primarily needed
	for updating the AUTHORS file
-	instead of executing the misc/sortac binary, its code was embedded
	into the updateac command itself

The modified updateac command will be added to x/build repository soon,
and the misc/sortac command can be removed afterwards.

Updates #12042

Change-Id: Ibf73068698b14b1aad4df4490747b488508ff5fd
Reviewed-on: https://go-review.googlesource.com/c/156278Reviewed-by: Filippo Valsorda <filippo@golang.org>