Fix the Mac toolchain definition in the GN build.

build/toolchain/gcc_toolchain.gni

Index: build/toolchain/gcc_toolchain.gni
diff --git a/build/toolchain/gcc_toolchain.gni b/build/toolchain/gcc_toolchain.gni
index 7e66c168bb6a159ad1bb1895c8d13f8e91c00b83..9e53ef667e049ae8334475373de738342b5df3ad 100644
--- a/build/toolchain/gcc_toolchain.gni
+++ b/build/toolchain/gcc_toolchain.gni
@@ -19,11 +19,11 @@ template("gcc_toolchain") {
     assert(defined(invoker.cc), "gcc_toolchain() must specify a \"cc\" value")
     assert(defined(invoker.cxx), "gcc_toolchain() must specify a \"cxx\" value")
     assert(defined(invoker.ar), "gcc_toolchain() must specify a \"ar\" value")
-    assert(defined(invoker.ar), "gcc_toolchain() must specify a \"ld\" value")
+    assert(defined(invoker.ld), "gcc_toolchain() must specify a \"ld\" value")
     assert(defined(invoker.toolchain_cpu_arch),
-                   "gcc_toolchain() must specify a \"toolchain_cpu_arch\"")
+           "gcc_toolchain() must specify a \"toolchain_cpu_arch\"")
     assert(defined(invoker.toolchain_os),
-                   "gcc_toolchain() must specify a \"toolchain_os\"")
+           "gcc_toolchain() must specify a \"toolchain_os\"")
 
     # We can't do string interpolation ($ in strings) on things with dots in
     # them. To allow us to use $cc below, for example, we create copies of