selftests/bpf: Allow macros in __retval

selftests/bpf: Allow macros in __retval

Allow macro expansion for values passed to the `__retval` and
`__retval_unpriv` attributes. This is especially useful for testing
programs which return various error codes.

With this change, the code for parsing special literals can be made
simpler, as the literals are defined via macros. The only exception is
INT_MIN which expands to (-INT_MAX -1), which is not single number and
cannot be parsed by strtol. So, we instead use a prefixed literal
_INT_MIN in __retval and handle it separately (assign the expected
return to INT_MIN). Also, strtol cannot handle the "ll" suffix so change
the value of POINTER_VALUE from 0xcafe4all to 0xbadcafe.

Signed-off-by: Viktor Malik <vmalik@redhat.com>
Link: https://lore.kernel.org/r/a6c6b551ae0575351faa7b7a1df52f9341a5cbe8.1750917800.git.vmalik@redhat.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>