[Android] Build goldfish kernel with NDK toolchain.

Here is the way to compile x86 goldfish and run on emulator

* AOSP Marshmallow + Goldfish 3.4

* ARCH=x86 (You don't need to set SUBARCH)

* Compiler version : gcc-4.8 (prebuilt x86 compiler from Android NDK)

  (Set CROSS_COMPILE environment variable)

$ make ARCH=x86 CC="${CROSS_COMPILE}gcc -mno-android" goldfish_defconfig

$ make ARCH=x86 CC="${CROSS_COMPILE}gcc -mno-android" -j40

And use 'arch/x86/boot/bzImage' as 'kernel-qemu' image

Please note that you SHOULD use '-mno-android' option to build linux kernel with NDK toolchains.

The reason is, '-mandroid' option is enabled by default at GCC in NDK(file name is something like *-*-android*)

Remember that NDK toolchain is used to build native executable or libraries run on Android devices.

And, in general, build environment of linux kernel is 'GCC' + 'GLIBC'.

(Note that, linux kernel binary even if it runs on Android device, it doesn't have any dependency on Android platform(ex. bionic or libc))

So, to build linux kernel, '-mandroid' option should be disabled by using '-mno-android' option.