selftests: bpf: Add tests for signed loads from arena

Add tests for loading 8, 16, and 32 bits with sign extension from arena,
also verify that exception handling is working correctly and correct
assembly is being generated by the x86 and arm64 JITs.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Link: https://lore.kernel.org/r/20250923110157.18326-4-puranjay@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>