build: Rename arm to arm32 and check if it's really supported

[real-or-random]

See the discussion starting here: #936 (comment)

[Psycho-Pirate]

I would like to work on this. Please tell me which files need to be updated.

[real-or-random]

Have a look at configure.ac. You need to be familiar with shell scripting, and you need to figure out a way to detect arm32 support. The autoconf documentation may be useful. (https://www.gnu.org/software/autoconf/)

[Psycho-Pirate]

Thanks, I will look into this!

[real-or-random]

@Psycho-Pirate Are you still interested in working on this?

[hebasto]

At least in CMake-based build system, it seems worth making the SECP256K1_ASM option actually dependent on the value of the CMAKE_SYSTEM_PROCESSOR variable. Then ON/OFF values will be enough for it with defaults as follows:

• ON for x86_64
• OFF for arm