devel/llvm19: prune build on 32-bit archs

Given trends in compiler resource use, it is incresingly implausible to
consider cross-development from a 32-bit system. As such, stop spending
time building LLVM bits are are vinishingly unlikely to be used on
32-bit systems.

Default to BE_NATIVE (no cross build support) on 32-bit plaforms.
Disable BE_AMDGPU and BE_WASM as well. BE_AMDGPU consumes enormous
amounts of memory. BE_WASM is less problematic, but no one has stood up
to say they need it so turn it off for now.

Completely disable MLIR and POLLY on 32-bit. Just building MLIR fails
routinly on armv7 and there aren't a lot of direct users (it's used by
FLANG, but FLANG is 64-bit only). Polly is pretty niche and adds quite
a bit of build time.

Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D46239