[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@357660 91177308-0d34-0410-b5e6-96231b3b80d8

utils/gn/build/toolchain/BUILD.gn

@@ -241,7 +241,7 @@ toolchain("win") {
   }
 
   tool("alink") {
-    command = "lib /nologo {{arflags}} /out:{{output}} {{inputs}}"
+    command = "$link /lib /nologo {{arflags}} /out:{{output}} {{inputs}}"
     description = "LIB {{output}}"
     outputs = [
       "{{output_dir}}/{{target_output_name}}.lib",