[builtins] Fix integer/pointer confusion in gcc_personality_v0.c

This fixes the implementation for architectures like CHERI with strong
pointer provenance (pointers, and thus uintptr_t, are represented as
hardware capabilities). Specifically, adding two uintptr_t's together
(as is done for `start + length` and `funcStart + landingPad`) has
ambiguous provenance, whereas using a plain integer (such as size_t) for
the offset operand does not. Also, readULEB128 is creating a plain
integer, not a pointer.

On all currently-supported architectures this should be an NFC, as
size_t and uintptr_t end up being the same underlying plain integer
type.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D95537