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

gn/BUILD.gn

Index: gn/BUILD.gn
diff --git a/gn/BUILD.gn b/gn/BUILD.gn
index 71a8259257b83b689e3d42379a0f64a94f84bf8c..e514bc3be4c4f5d154c9665482cb36310b76a6d2 100644
--- a/gn/BUILD.gn
+++ b/gn/BUILD.gn
@@ -62,6 +62,7 @@ toolchain("gcc_like") {
     outputs = [
       "{{source_out_dir}}/{{target_output_name}}.{{source_name_part}}.o",
     ]
+    description = "$cc ... -o {{output}}"
   }
 
   tool("cxx") {
@@ -71,6 +72,7 @@ toolchain("gcc_like") {
     outputs = [
       "{{source_out_dir}}/{{target_output_name}}.{{source_name_part}}.o",
     ]
+    description = "$cxx ... -o {{output}}"
   }
 
   tool("asm") {
@@ -80,6 +82,7 @@ toolchain("gcc_like") {
     outputs = [
       "{{source_out_dir}}/{{target_output_name}}.{{source_name_part}}.o",
     ]
+    description = "$cc ... -o {{output}}"
   }
 
   tool("alink") {
@@ -89,6 +92,7 @@ toolchain("gcc_like") {
     ]
     default_output_extension = ".a"
     output_prefix = "lib"
+    description = "$ar {{output}} ..."
   }
 
   tool("solink") {
@@ -105,6 +109,7 @@ toolchain("gcc_like") {
     ]
     output_prefix = "lib"
     default_output_extension = ".so"
+    description = "$cxx -shared ... -o {{output}}"
   }
 
   tool("link") {
@@ -112,6 +117,7 @@ toolchain("gcc_like") {
     outputs = [
       "{{root_out_dir}}/{{target_output_name}}{{output_extension}}",
     ]
+    description = "$cxx ... -o {{output}}"
   }
 
   tool("stamp") {