Enable LLD for POSIX LTO builds on Linux.

build/toolchain/toolchain.gni

Index: build/toolchain/toolchain.gni
diff --git a/build/toolchain/toolchain.gni b/build/toolchain/toolchain.gni
index 6eb3c9080393b1b80c79650257bcdd0b5dd745d9..460078aa0e29366ab0b9136d8d02891831e913ce 100644
--- a/build/toolchain/toolchain.gni
+++ b/build/toolchain/toolchain.gni
@@ -16,12 +16,6 @@ declare_args() {
   allow_posix_link_time_opt = target_os == "linux" && !is_chromeos &&
                               target_cpu == "x64" && is_official_build
 
-  # Set to true to use lld, the LLVM linker. This flag may be used on Windows
-  # with the shipped LLVM toolchain, or on Linux with a self-built top-of-tree
-  # LLVM toolchain (see llvm_force_head_revision in
-  # build/config/compiler/BUILD.gn).
-  use_lld = is_win && host_os != "win"
-
   # If used with allow_posix_link_time_opt, it enables the experimental support
   # of ThinLTO that links 3x-10x faster but (as of now) does not have all the
   # important optimizations such us devirtualization implemented. See also
@@ -52,6 +46,12 @@ declare_args() {
     # Clang compiler version. Clang files are placed at version-dependent paths.
     clang_version = "5.0.0"
   }
+
+  # Set to true to use lld, the LLVM linker. This flag may be used on Windows
+  # or Linux.
+  use_lld = (is_win && host_os != "win") ||
+            (allow_posix_link_time_opt && target_os == "linux" &&
+             !is_chromeos && target_cpu == "x64")
 }
 
 # Check target_os here instead of is_ios as this file is loaded for secondary