selftests/bpf: add tests for recent bugfixes

These tests should cover the following cases:

 - MOV with both zero-extended and sign-extended immediates
 - implicit truncation of register contents via ALU32/MOV32
 - implicit 32-bit truncation of ALU32 output
 - oversized register source operand for ALU32 shift
 - right-shift of a number that could be positive or negative
 - map access where adding the operation size to the offset causes signed
   32-bit overflow
 - direct stack access at a ~4GiB offset

Also remove the F_LOAD_WITH_STRICT_ALIGNMENT flag from a bunch of tests
that should fail independent of what flags userspace passes.

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>