Return gmake as the default name for GNU make on NetBSD

Summary:
The base make(1) on NetBSD is BSD make.

In the default installation of NetBSD GNU make comes via pkgsrc under the gmake name.

Reviewers: emaste, tfiala, clayborg

Subscribers: joerg, lldb-commits

Differential Revision: http://reviews.llvm.org/D15261

llvm-svn: 254947