More explicit local variable names in ada_value_primitive_packed_val

More explicit local variable names in ada_value_primitive_packed_val

A number of local variables declared in ada_value_primitive_packed_val
have a name that could, IMO, be improved to, either: Be more explicit
about what the variable is about (Eg: "src" is an index, so rename it
to "src_idx"); or be more consistent with other variables that they
relate to: for instance, several variables refer to the source via
"src" (Eg: srcBitsLeft, nsrc), but the buffer they refer to is called
"bytes", so patch renames "bytes" to "src".

This should help read and understand a little more easily the code
inside this function.  No real code change otherwise.

gdb/ChangeLog:

        * ada-lang.c (ada_value_primitive_packed_val): Make the name
        of various local variables more explicit and consistent.
        No real code change otherwise.