build: detect and compile arm32 assembly #1751

Currently, arm32 assembly gets only compiled if explicitly set via SECP256K1_ASM=arm32. When SECP256K1_ASM is set to AUTO, then the detection script only checks for x86 assembly. I propose adding check_arm32_assembly() also in the AUTO branch. see https://github.com/bitcoin/bitcoin/issues/32832#issuecomment-3268056766.

This is the current snippet for reference:

set(SECP256K1_ASM "AUTO" CACHE STRING "Assembly to use: \"AUTO\", \"OFF\", \"x86_64\" or \"arm32\" (experimental). [default=AUTO]")
set_property(CACHE SECP256K1_ASM PROPERTY STRINGS "AUTO" "OFF" "x86_64" "arm32")
check_string_option_value(SECP256K1_ASM)
if(SECP256K1_ASM STREQUAL "arm32")
  enable_language(ASM)
  include(CheckArm32Assembly)
  check_arm32_assembly()
  if(HAVE_ARM32_ASM)
    add_compile_definitions(USE_EXTERNAL_ASM=1)
  else()
    message(FATAL_ERROR "ARM32 assembly requested but not available.")
  endif()
elseif(SECP256K1_ASM)
  include(CheckX86_64Assembly)
  check_x86_64_assembly()
  if(HAVE_X86_64_ASM)
    set(SECP256K1_ASM "x86_64")
    add_compile_definitions(USE_ASM_X86_64=1)
  elseif(SECP256K1_ASM STREQUAL "AUTO")
    set(SECP256K1_ASM "OFF")
  else()
    message(FATAL_ERROR "x86_64 assembly requested but not available.")
  endif()
endif()

Currently, arm32 assembly gets only compiled if explicitly set via SECP256K1_ASM=arm32. When SECP256K1_ASM is set to AUTO, then the detection script only checks for x86 assembly. I propose adding check_arm32_assembly() also in the AUTO branch. see bitcoin/bitcoin#32832 (comment).

The comment from @sipa that you’re referring to clearly states:

because they were ... unreviewed @Raimo33

If this situation has changed and the assembly code has been reviewed, please leave references here. Until then, I don’t see any reason to bring this into Bitcoin Core.

doesn't the CI test secp functions? both in Bitcoin core and here.