At least the u-boot project doesn't seem to have an issue with that since 2011.
http://patchwork.ozlabs.org/patch/123541/
http://git.denx.de/cgi-bin/gitweb.cgi?p=u-boot.git;a=blob;f=include/compiler.h;h=0734ed494274558a0656925220d7e4861da7231e;hb=master
But if you insist, then let's use strtoull() and put a comment to it.
Btw. `scanf()` supports `uintptr_t` with `SCNuPTR`
http://stackoverflow.com/questions/5795978/string-format-for-intptr-t-and-uintptr-t