[gn] Use "$link /lib" for archives instead of lib.exe

Summary:
This avoids the need to talk about lib.exe or llvm-lib.exe and it does
the right thing with LLD.

Reviewers: thakis

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60155

llvm-svn: 357660