(riscv/ada) fix error when calling functions with range argument

(riscv/ada) fix error when calling functions with range argument

Using the gdb.ada/call_pn.exp testcase, and running it by hand on
riscv64-elf, we get the following error:

    (gdb) call pn(55)
    Could not compute alignment of type

The problem occurs because the parameter's type is a TYPE_CODE_RANGE,
and that type code is not handled by riscv_type_alignment. So this patch
fixes the issue by handling TYPE_CODE_RANGE the same way we handle other
integral types.

gdb/ChangeLog:

        * riscv-rdep.c (riscv_type_alignment): Handle TYPE_CODE_RANGE.

Tested on riscv64-elf using AdaCore's testsuite.