Add buffer append method for {,u}intptr_t

[nwf-msr]

Otherwise, on platforms for which {,u}intptr_t aren't just typedef-s of
other scalar types, it's ambiguous which way an implicit cast should go.

[nwf-msr]

With this and the SNMALLOC_LINK_ICF=OFF hack, all four of {debug, release} x {hybrid, purecap} build in Morello CI.

[davidchisnall]

I have a similar fix locally to this that casts to void* in both cases. This is probably closer to the right thing because then we can print the full capability in the void* case.

[mjp41]

LGTM