Enable SIMD for armv7

Summary:
@public

This diff makes the gradle build system use SIMD for mozjpeg for armv7 architectures.

**Testplan: files used in build**

```
[user:spectrum]$ ./gradlew clean && ./gradlew prepareNative && ./gradlew assembleDebug

...

> Task :android:spectrumpluginjpeg:generateJsonModelDebug
Variant=debug ABI=armeabi-v7a :-- CMAKE_SYSTEM_PROCESSOR=armv7-a
Variant=debug ABI=armeabi-v7a :-- Building mozjpeg with armv7-a SIMD support
Variant=debug ABI=armeabi-v7a :-- Configuring done
Variant=debug ABI=armeabi-v7a :-- Generating done
Variant=debug ABI=armeabi-v7a :-- Build files have been written to: /Users/hugenroth/fbsource/xplat/spectrum/android/spectrumpluginjpeg/.externalNativeBuild/cmake/debug/armeabi-v7a
Variant=debug ABI=arm64-v8a :-- CMAKE_SYSTEM_PROCESSOR=aarch64
Variant=debug ABI=arm64-v8a :-- Building mozjpeg without SIMD support
Variant=debug ABI=arm64-v8a :-- Configuring done
Variant=debug ABI=arm64-v8a :-- Generating done
Variant=debug ABI=arm64-v8a :-- Build files have been written to: /Users/hugenroth/fbsource/xplat/spectrum/android/spectrumpluginjpeg/.externalNativeBuild/cmake/debug/arm64-v8a
Variant=debug ABI=x86 :-- CMAKE_SYSTEM_PROCESSOR=i686
Variant=debug ABI=x86 :-- Building mozjpeg without SIMD support
Variant=debug ABI=x86 :-- Configuring done
Variant=debug ABI=x86 :-- Generating done
Variant=debug ABI=x86 :-- Build files have been written to: /Users/hugenroth/fbsource/xplat/spectrum/android/spectrumpluginjpeg/.externalNativeBuild/cmake/debug/x86
Variant=debug ABI=x86_64 :-- CMAKE_SYSTEM_PROCESSOR=x86_64
Variant=debug ABI=x86_64 :-- Building mozjpeg without SIMD support
Variant=debug ABI=x86_64 :-- Configuring done
Variant=debug ABI=x86_64 :-- Generating done
Variant=debug ABI=x86_64 :-- Build files have been written to: /Users/hugenroth/fbsource/xplat/spectrum/android/spectrumpluginjpeg/.externalNativeBuild/cmake/debug/x86_64

...

Build spectrumpluginjpeg armeabi-v7a
[1/155] Building C object ../../../../build/mozjpeg/armeabi-v7a/CMakeFiles/mozjpeg.dir/jcapimin.c.o
[2/155] Building C object ../../../../build/mozjpeg/armeabi-v7a/CMakeFiles/mozjpeg.dir/jcapistd.c.o
[3/155] Building C object ../../../../build/mozjpeg/armeabi-v7a/CMakeFiles/mozjpeg.dir/jccoefct.c.o

...

[51/155] Building C object ../../../../build/mozjpeg/armeabi-v7a/CMakeFiles/mozjpeg.dir/jmemnobs.c.o
[52/155] Building C object ../../../../build/mozjpeg/armeabi-v7a/CMakeFiles/mozjpeg.dir/simd/jsimd_arm.c.o
[53/155] Building ASM object ../../../../build/mozjpeg/armeabi-v7a/CMakeFiles/mozjpeg.dir/simd/jsimd_arm_neon.S.o
```

**Testplan: correct result in sample app**

{F148119273}

Reviewed By: passy

Differential Revision: D13609467

fbshipit-source-id: 132ea5342c73d00a5e51a92784a8d8809250b176

androidLibs/third-party/mozjpeg/override/CMakeLists.txt

@@ -1,7 +1,10 @@
 cmake_minimum_required(VERSION 3.6.0)
 project(mozjpeg C)
 
-add_library(mozjpeg STATIC
+set(can_use_assembler TRUE)
+enable_language(ASM)
+
+list(APPEND mozjpegsources
   jcapimin.c
   jcapistd.c
   jccoefct.c
@@ -53,9 +56,24 @@ add_library(mozjpeg STATIC
   jdarith.c
   transupp.c
   jmemnobs.c
-  jsimd_none.c
 )
 
+if(${CMAKE_SYSTEM_PROCESSOR} STREQUAL "armv7-a")
+  message(STATUS "Building mozjpeg with armv7-a SIMD support")
+  list(APPEND mozjpegsources
+    simd/jsimd_arm.c
+    simd/jsimd_arm_neon.S
+  )
+else()
+  message(STATUS "Building mozjpeg without SIMD support")
+  list(APPEND mozjpegsources
+    jsimd_none.c
+  )
+endif()
+
+
+add_library(mozjpeg STATIC ${mozjpegsources})
+
 target_compile_options(mozjpeg PRIVATE
   -fno-omit-frame-pointer
   -fexceptions