gdb: have value_as_address call unpack_pointer

gdb: have value_as_address call unpack_pointer

While refactoring some other code in gdb/python/* I wanted to merge
two code paths.  One path calls value_as_address, while the other
calls unpack_pointer.

I suspect calling value_as_address is the correct choice, but, while
examining the code I noticed that value_as_address calls unpack_long
rather than unpack_pointer.

Under the hood, unpack_pointer does just call unpack_long so there's
no real difference here, but it feels like value_as_address should
call unpack_pointer.

I've updated the code to use unpack_pointer, and changed a related
comment to say that we call unpack_pointer.  I've also adjusted the
header comment on value_as_address.  The existing header refers to
some code that is now commented out.

Rather than trying to describe the whole algorithm of
value_as_address, which is already well commented within the function,
I've just trimmed the comment on value_as_address to be a brief
summary of what the function does.

There should be no user visible changes after this commit.

Reviewed-By: Tom Tromey <tom@tromey.com>