Fix cross build bug for aarch64 with simd-accel

[kings-way]

This patch has done 3 things:

1. Fix cross build bug for aarch64 with simd-accel
   The current logic in build.rs #[cfg(target_arch = "x86_64")] will fail when cross build for aarch64.
   As the build.rs will be compiled to a binary on host, the target_arch value will be set to host architecture.
   Instead, we should read target_arch from environment. Reproduce it on a x86_64 linux host:
   cargo build --target=aarch64-unknown-linux-gnu --features=simd-accel -vv

2. Set -march=armv8-a as default value for aarch64
   I think we should set a default -march value for aarch64, and this has been tested for aarch64-linux and aarch64-darwin (Apple M1).

3. Enable aarch64 + simd-accel for Android build
   Enable simd-accel feature for android, and maybe we can also do it for iOS?

I don't know much about MacOS and iOS, please correct me if there is any mistake~
releated: #98 #95 #92

[nazar-pc]

1. This is a good catch, thanks!
2. Is this actually beneficial in any way? I don't see a significant reason to include it unless it is a major boost and definitely doesn't cause any issues for users of this library, you can still force it to whatever you want with an environment variable
3. Have you seen significant improvement with SIMD version on Android?

[kings-way]

Hi @nazar-pc, in regard to your 2nd and 3rd questions, I did some tests just now. And the answer is ... no.
These benchmarks ran on Qualcomm Snapdragon 865 running Android 11, with both gcc 10 and gcc 4.9 (from android NDK)

simd-accel	-march	benchmark
Enabled	armv8.2-a	630MB/s
Enabled	armv8-a	630MB/s
Enabled	none	630MB/s
Disabled	-	630MB/s

It seems that the C compiler (gcc here) has already done it's job for aarch64. No need to set -march option.
So I am going to do another force push to my master branch, only do the bug fix.

[nazar-pc]

Can you also simplify that match to a single line it was before, bump patch version and add a changelog entry similar to existing ones so I can make a new release, please?

[kings-way]

Hi, put the match or if statement in one single line brings us nothing but ugly code.
if "x86_64" == env::var("CARGO_CFG_TARGET_ARCH").unwrap().as_str() { build.flag(&"-march=haswell"); }

As for the version and changelog, I have pushed some newer commits.
Let me know if there is anything missed.

[nazar-pc]

Looks good, thanks!