[Ada] Error adding/subtracting pointer value to/from integral.

[Ada] Error adding/subtracting pointer value to/from integral.

When trying to evaluate an expression which adds a pointer and
an integral, the evaluation succeeds if the pointer is on
the left handside of the operator, but not when it is on the right
handside:

    (gdb) p something'address + 0
    $1 = (system.address) 0x613418 <pck.something>
    (gdb) p 0 + something'address
    Argument to arithmetic operation not a number or boolean.

Same issue when doing subtractions:

    (gdb) p something'address - 0
    $2 = (system.address) 0x613418 <pck.something>
    (gdb) p 0 - something'address
    Argument to arithmetic operation not a number or boolean.

This patch enhances the Ada expression evaluator to handle
these two situations.

gdb/ChangeLog:

        * ada-lang.c (ada_evaluate_subexp) <BINOP_ADD>: Add handling
        of the case where the second operand is a pointer.
        <BINOP_SUB>: Likewise.

gdb/testsuite/ChangeLog:

        * gdb.ada/addr_arith: New testcase.

Tested on x86_64-linux.