Use API 16 for armv7, x86 as current react-native spec (#66)

Summary:
    While building arm7, x86, use API 16.
    While building arm64, x86_64, use API 21.
    (Android introduced 64 bits support after API 21).

    There are some patches for missing symbols from NDK API 16.
    1. log2() is supported until API 18.
       Solved this by introduced a log2() polyfill.

    2. getline() is supported until API 18.
       As the WebKit WTF code is not being used in JSC, just comment out it.
       For discussion details, see the PR commants:
       #66 (comment)

    3. Add '-Wl,--no-undefined' ldflags to prevent missing NDK symbols in the future.

lib/android-jsc/build.gradle

@@ -4,7 +4,7 @@ android {
     compileSdkVersion 28
 
     defaultConfig {
-        minSdkVersion 21
+        minSdkVersion 16
         targetSdkVersion 28
         versionCode 1
         versionName "1.0"

patches/jsc.patch

@@ -291,3 +291,43 @@ diff -aur target-org/webkit/Source/JavaScriptCore/CMakeLists.txt target/webkit/S
  )
 +add_definitions(-DJSC_VERSION="${JSC_VERSION}")
 
+diff -aur target-org/webkit/Source/WTF/wtf/linux/MemoryFootprintLinux.cpp target/webkit/Source/WTF/wtf/linux/MemoryFootprintLinux.cpp
+--- target-org/webkit/Source/WTF/wtf/linux/MemoryFootprintLinux.cpp     2018-11-11 23:05:48.000000000 +0800
++++ target/webkit/Source/WTF/wtf/linux/MemoryFootprintLinux.cpp 2018-11-12 23:39:22.000000000 +0800
+@@ -23,6 +23,10 @@
+  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+  */
+
++// CUSTOMIZE_REACT_NATIVE: getline() does not implemented until Android NDK API 18.
++// Since MemoryFootprint does not being used in JSC, comment out to prevent build break.
++#if !defined(CUSTOMIZE_REACT_NATIVE)
++
+ #include "config.h"
+ #include "MemoryFootprint.h"
+
+@@ -107,3 +111,4 @@
+ }
+
+ }
++#endif // !defined(CUSTOMIZE_REACT_NATIVE)
+diff -aur target-org/webkit/Source/JavaScriptCore/Sources.txt target/webkit/Source/JavaScriptCore/Sources.txt
+--- target-org/webkit/Source/JavaScriptCore/Sources.txt 2018-11-11 23:05:40.000000000 +0800
++++ target/webkit/Source/JavaScriptCore/Sources.txt     2018-11-12 00:03:26.000000000 +0800
+@@ -1046,3 +1046,6 @@
+
+ // Derived Sources
+ yarr/YarrCanonicalizeUnicode.cpp
++
++// Polyfills
++polyfills/log2.cpp
+diff -aur /dev/null target/webkit/Source/JavaScriptCore/polyfills/log2.cpp
+--- /dev/null   2018-11-12 01:21:57.000000000 +0800
++++ target/webkit/Source/JavaScriptCore/polyfills/log2.cpp      2018-11-12 01:19:49.000000000 +0800
+@@ -0,0 +1,7 @@
++#include <math.h>
++
++#if defined(__ANDROID__) && __ANDROID_API__ < 18
++double log2(double x) {
++  return log(x) / log(2.0);
++}
++#endif

scripts/compile/all.sh

@@ -21,13 +21,15 @@ compile_arch() {
 compile() {
   for arch in arm x86
   do
+    export ANDROID_API=$ANDROID_API_FOR_ABI_32
     export JSC_ARCH=$arch
     export ENABLE_COMPAT=1
     compile_arch
   done
 
   for arch in arm64 x86_64
   do
+    export ANDROID_API=$ANDROID_API_FOR_ABI_64
     export JSC_ARCH=$arch
     export ENABLE_COMPAT=0
     compile_arch

scripts/compile/common.sh

@@ -110,7 +110,8 @@ process_switch_options "INTL"
 
 # checks
 err=false
-if ! [[ $ANDROID_API ]]; then echo "set ANDROID_API to the minimum supported Android platform version (e.g. 15)"; err=true; fi
+if ! [[ $ANDROID_API_FOR_ABI_32 ]]; then echo "set ANDROID_API_FOR_ABI_32 to the minimum supported Android platform version for arm and x86 (e.g. 16)"; err=true; fi
+if ! [[ $ANDROID_API_FOR_ABI_64 ]]; then echo "set ANDROID_API_FOR_ABI_64 to the minimum supported Android platform version for arm64 and x86_64 (e.g. 21)"; err=true; fi
 if ! [[ $FLAVOR ]]; then echo "set FLAVOR to the name of the flavor"; err=true; fi
 if ! [[ $CROSS_COMPILE_PLATFORM ]]; then echo "set JSC_ARCH to one of {arm,arm64,x86,x86_64}"; err=true; fi
 if ! [[ $ANDROID_HOME ]]; then echo "set ANDROID_HOME to android sdk dir"; err=true; fi
@@ -127,6 +128,7 @@ COMMON_LDFLAGS=" \
 -Wl,--gc-sections \
 -Wl,--exclude-libs,libgcc.a \
 -Wl,--exclude-libs,libunwind.a \
+-Wl,--no-undefined \
 "
 
 COMMON_CFLAGS=" \
@@ -141,6 +143,7 @@ COMMON_CFLAGS=" \
 -fPIC \
 -fvisibility=hidden \
 -DNDEBUG \
+-DCUSTOMIZE_REACT_NATIVE \
 $SWITCH_COMMON_CFLAGS_INTL \
 "
 

scripts/start.sh

@@ -1,6 +1,7 @@
 #!/bin/bash -e
 
-export ANDROID_API=21
+export ANDROID_API_FOR_ABI_32=16
+export ANDROID_API_FOR_ABI_64=21
 export ROOTDIR=$PWD
 export TARGETDIR=$ROOTDIR/build/target
 source $ROOTDIR/scripts/info.sh