Cherry pick 661802 to fix 64bit arm build

Remove default cortex-a8 tuning.

Probably not even the dominant platform the library is being built for.
Add --cpu= option description to help. The option already exists.

Don't allow passing just --cpu as a no-op.

BUG=826

Change-Id: Iaa3f4f693ec78b18927b159b480daafeba0549c0

build/make/configure.sh

@@ -511,9 +511,8 @@ process_common_cmdline() {
         ;;
         --force-target=*) toolchain="${toolchain:-${optval}}"; enable_feature force_toolchain
         ;;
-        --cpu)
-        ;;
-        --cpu=*) tune_cpu="$optval"
+        --cpu=*)
+        tune_cpu="$optval"
         ;;
         --extra-cflags=*)
         extra_cflags="${optval}"

configure

@@ -26,6 +26,7 @@ Advanced options:
   ${toggle_unit_tests}            unit tests
   ${toggle_decode_perf_tests}     build decoder perf tests with unit tests
   ${toggle_encode_perf_tests}     build encoder perf tests with unit tests
+  --cpu=CPU                       tune for the specified CPU (ARM: cortex-a8, X86: sse3)
   --libc=PATH                     path to alternate libc
   --size-limit=WxH                max size to allow in the decoder
   --as={yasm|nasm|auto}           use specified assembler [auto, yasm preferred]