Fix: Pointers are rejected by integer element compile time assertion for array and...

Fix: Pointers are rejected by integer element compile time assertion for array and sequence

commit 2df82195d140b ("Add compile time assertion that array and
sequence have integer elements") introduced a check to validate that
sequences and arrays only contain integers. This was meant to refuse
arrays of double/float which are not supported.

However, as a side-effect, this also refuses arrays and sequences of
pointers, which were accepted prior to lttng-ust 2.13.

Introduce a lttng_ust_is_pointer_type() and use it in the array/sequence
type validation. The trick here is to use the fact that a difference
between two pointers in C is an integer. Therefore, we can validate that
an argument type is a pointer similarly to C++ is_pointer.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Fixes: #1355
Change-Id: I7c96d24ab68fb711f85eccdb781a3c513b45c5dc