monitor: Prevent sign-extension of 32-bit addresses printed by info mem

Previously, on 32-bit i386, info mem used signed 32-bit int's to store
the page table indexes.  As a result, address calculation was done in
32 bits and then incorrectly sign-extended to 64 bits, yielding output
like

ffffffffef000000-ffffffffef031000 0000000000031000 ur-
ffffffffef7bc000-ffffffffef7bd000 0000000000001000 urw
ffffffffef7bd000-ffffffffef7be000 0000000000001000 ur-

This makes these indexes unsigned, which yields correct output

00000000ef000000-00000000ef031000 0000000000031000 ur-
00000000ef7bc000-00000000ef7bd000 0000000000001000 urw
00000000ef7bd000-00000000ef7be000 0000000000001000 ur-

Signed-off-by: Austin Clements <amdragon@mit.edu>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
