Can boringssl work on ARMv8 bare metal platform? I tried build boringssl with aarch64-elf-gcc, but it refused to build.
If it does, any porting guide or suggestions?
Probably not out of the box. But you should probably not even try using it, mainly because, according to Google itself, it is not intended for general use.
This is never good to be on your own when using a library, more specifically a cryptographic one. This is usually synonym for no bug fixes, no support, no user forums among other things.
You could rather consider a library that was designed for this purpose, such as mbedtls (formerly known as PolarSSL).
It is being used on a wide range of systems, from bare-metal systems (FreeRTOS) to Linux (The Hiawhata web server does use it for example).
Update: Even if support for Armv8-a hardware crypto extensions is needed, you could still reuse BoringSSL Armv8-a optimized routines (ISC license) or the Cavium armv8_crypto library (BSD license), to replace mbedtls (Apache 2.0 lisense) equivalent routines: cryptographic functions usually have clean and small interfaces.
From my experience, this may still be faster than porting a library targeting a general purpose operating system if your target is a bare-metal one, but you ultimately have to evaluate the costs for both options in your specific case.
My guess would be that there is far less work involved for adding support for Armv8-a crypto extensions to mbedtls using already existing, supported code under the proper license, than attempting to strip-down openssl or boringssl for use on a bare-metal target.
There is a very good piece of documentation explaining how to add support for hardware-accelerated crypto to mbedtls here, this may help you evaluating your options.