Add support for a clang toolchain on ppc64

This change allows a clang toolchain to be used on ppc64 hosts.

This change is part of the effort to upstream a set of patches
that allow chromium to build and run on ppc64le hosts available here:
https://wiki.raptorcs.com/wiki/Porting/Chromium

Change-Id: I4496523f4948c675d4d1d8e529e0f4d5bbf99b8c
Reviewed-on: https://chromium-review.googlesource.com/c/1480698
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635290}

AUTHORS

@@ -808,6 +808,7 @@ ShankarGanesh K <blr.bmlab@gmail.com>
 Shanmuga Pandi M <shanmuga.m@samsung.com>
 Shaobo Yan <shaobo.yan@intel.com>
 Shashi Kumar <sk.kumar@samsung.com>
+Shawn Anastasio <shawnanastasio@gmail.com>
 Shen Yu <shenyu.tcv@gmail.com>
 Sherry Mou <wenjinm@amazon.com>
 Shez Baig <sbaig1@bloomberg.net>

build/toolchain/linux/BUILD.gn

@@ -5,6 +5,14 @@
 import("//build/config/sysroot.gni")
 import("//build/toolchain/gcc_toolchain.gni")
 
+clang_toolchain("clang_ppc64") {
+  enable_linker_map = true
+  toolchain_args = {
+    current_cpu = "ppc64"
+    current_os = "linux"
+  }
+}
+
 clang_toolchain("clang_arm") {
   toolprefix = "arm-linux-gnueabihf-"
   toolchain_args = {