Describe GCC-like asm steps as "assemble ..."

Oddly, the MSVC toolchain already says "assemble ...".

Change-Id: Iabf06afa1b7c05b5699a26813f1469346be439a5
Reviewed-on: https://skia-review.googlesource.com/14367
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>

gn/BUILD.gn

@@ -668,7 +668,7 @@ toolchain("gcc_like") {
     outputs = [
       "{{source_out_dir}}/{{target_output_name}}.{{source_name_part}}.o",
     ]
-    description = "compile {{source}}"
+    description = "assemble {{source}}"
   }
 
   tool("alink") {