Add a script to bootstrap GN from source.

tools/gn/bootstrap/build.ninja.template

Index: tools/gn/bootstrap/build.ninja.template
diff --git a/tools/gn/bootstrap/build.ninja.template b/tools/gn/bootstrap/build.ninja.template
new file mode 100644
index 0000000000000000000000000000000000000000..2ef5301e328a5a2e31437a6a33b777b86c8fd221
--- /dev/null
+++ b/tools/gn/bootstrap/build.ninja.template
@@ -0,0 +1,25 @@
+cc = cc
+cxx = c++
+ld = $cc
+ldxx = $cxx
+ar = ar
+
+rule cc
+  command = $cc -MMD -MF $out.d $defines $includes $cflags $cflags_c -c $in -o $out
+  description = CC $out
+  depfile = $out.d
+  deps = gcc
+
+rule cxx
+  command = $cxx -MMD -MF $out.d $defines $includes $cflags $cflags_cc -c $in -o $out
+  description = CXX $out
+  depfile = $out.d
+  deps = gcc
+
+rule alink_thin
+  command = rm -f $out && $ar rcsT $out $in
+  description = AR $out
+
+rule link
+  command = $ld $ldflags -o $out -Wl,--start-group $in $solibs -Wl,--end-group $libs
+  description = LINK $out