Stop concealing pointer provenance in phn_link_get

At least for LLVM, [casting from an integer to a pointer hides provenance information](https://clang.llvm.org/extra/clang-tidy/checks/performance/no-int-to-ptr.html)
and inhibits optimizations. Here's a [Godbolt link](https://godbolt.org/z/5bYPcKoWT)
showing how this change removes a couple unnecessary branches in
`phn_merge_siblings`, which is a very hot function. Canary profiles show
only minor improvements (since most of the cost of this function is in
cache misses), but there's no reason we shouldn't take it.

include/jemalloc/internal/ph.h

@@ -73,7 +73,7 @@ struct ph_s {
 
 JEMALLOC_ALWAYS_INLINE phn_link_t *
 phn_link_get(void *phn, size_t offset) {
-	return (phn_link_t *)(((uintptr_t)phn) + offset);
+	return (phn_link_t *)(((char *)phn) + offset);
 }
 
 JEMALLOC_ALWAYS_INLINE void