ensure ctx is used

Rather that (void)ing ctx, document the case where it might not be used.

BUG=webm:1612

Change-Id: I1f1ba9a3d52b43a6987dbe3afec96fa17101e3bf
diff --git a/vp8/common/generic/systemdependent.c b/vp8/common/generic/systemdependent.c
index 76c34d1..75ce7ef 100644
--- a/vp8/common/generic/systemdependent.c
+++ b/vp8/common/generic/systemdependent.c
@@ -88,8 +88,6 @@
 void vp8_machine_specific_config(VP8_COMMON *ctx) {
 #if CONFIG_MULTITHREAD
   ctx->processor_core_count = get_cpu_count();
-#else
-  (void)ctx;
 #endif /* CONFIG_MULTITHREAD */
 
 #if VPX_ARCH_ARM
@@ -98,5 +96,8 @@
   ctx->cpu_caps = x86_simd_caps();
 #elif VPX_ARCH_PPC
   ctx->cpu_caps = ppc_simd_caps();
+#else
+  // generic-gnu targets.
+  ctx->cpu_caps = 0;
 #endif
 }