Move uinteger_pow gdb/valarith.c to gdb/utils.c and make it public

This is a generic function which I would like to use in a followup
patch adding support for fixed-point types. So this commit moves it
out of valarith.c into util.c, and makes it non-static.

gdb/ChangeLog:

        * utils.h (uinteger_pow): Add declaration.
        * utils.c (uinteger_pow): Moved here (without changes)...
        * valarith.c (uinteger_pow): ... from here.