GN: add abbreviated versions of the command line as descriptions.

This should help make the bot logs a little easier to read.  (On the console
this wasn't much of a big deal, as Ninja auto-abbreviates when printing to an
interactive console.)

As usual, Ninja will print the full command when a task fails.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2188263002

Review-Url: https://codereview.chromium.org/2188263002
diff --git a/gn/BUILD.gn b/gn/BUILD.gn
index 71a8259..e514bc3 100644
--- a/gn/BUILD.gn
+++ b/gn/BUILD.gn
@@ -62,6 +62,7 @@
     outputs = [
       "{{source_out_dir}}/{{target_output_name}}.{{source_name_part}}.o",
     ]
+    description = "$cc ... -o {{output}}"
   }
 
   tool("cxx") {
@@ -71,6 +72,7 @@
     outputs = [
       "{{source_out_dir}}/{{target_output_name}}.{{source_name_part}}.o",
     ]
+    description = "$cxx ... -o {{output}}"
   }
 
   tool("asm") {
@@ -80,6 +82,7 @@
     outputs = [
       "{{source_out_dir}}/{{target_output_name}}.{{source_name_part}}.o",
     ]
+    description = "$cc ... -o {{output}}"
   }
 
   tool("alink") {
@@ -89,6 +92,7 @@
     ]
     default_output_extension = ".a"
     output_prefix = "lib"
+    description = "$ar {{output}} ..."
   }
 
   tool("solink") {
@@ -105,6 +109,7 @@
     ]
     output_prefix = "lib"
     default_output_extension = ".so"
+    description = "$cxx -shared ... -o {{output}}"
   }
 
   tool("link") {
@@ -112,6 +117,7 @@
     outputs = [
       "{{root_out_dir}}/{{target_output_name}}{{output_extension}}",
     ]
+    description = "$cxx ... -o {{output}}"
   }
 
   tool("stamp") {