selftests/rseq: Implement rseq_unqual_scalar_typeof

Allow defining variables and perform cast with a typeof which removes
the volatile and const qualifiers.

This prevents declaring a stack variable with a volatile qualifier
within a macro, which would generate sub-optimal assembler.

This is imported from the "librseq" project.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>