Disable sve vectorization at configure time, and don't set -march=native

[haampie]

1. This branch is hit at compile time:

   bricklib/stencils/cpuvfold.h

   Line 27 in d817250

   #error "This is for SVE512"

   which is rather annoying. Can you test whether 512 vector width is supported in CMake, and disable it if unsupported? Otherwise the build simply fails.
2. Can you please make this here

   bricklib/CMakeLists.txt

   Lines 105 to 118 in d817250

   	set(BRICK_BUILD_ARCH "native" CACHE STRING "Specify build architecture target")
   	check_cxx_compiler_flag("-march=${BRICK_BUILD_ARCH}" HAS_MARCH)
   	if (HAS_MARCH)
   	message(STATUS "Using march=${BRICK_BUILD_ARCH}")
   	add_compile_options(-march=${BRICK_BUILD_ARCH})
   	else ()
   	# Work around Summit only have mcpu flag
   	check_cxx_compiler_flag("-mcpu=${BRICK_BUILD_ARCH}" HAS_MCPU)
   	if (HAS_MCPU)
   	message(STATUS "Using mcpu=${BRICK_BUILD_ARCH}")
   	add_compile_options(-mcpu=${BRICK_BUILD_ARCH})
   	endif ()
   	endif ()

   opt in? Compiler wrappers may set the relevant -march flag, so in your CMake logic you can just test what vectorization features are supported without any additional flags and work with that.