Support --with-lg-page values larger than actual page size.

During over-allocation in preparation for creating aligned mappings,
allocate one more page than necessary if PAGE is the actual page size,
so that trimming still succeeds even if the system returns a mapping
that has less than PAGE alignment.  This allows compiling with e.g. 64
KiB "pages" on systems that actually use 4 KiB pages.

Note that for e.g. --with-lg-page=21, it is also necessary to increase
the chunk size (e.g. --with-malloc-conf=lg_chunk:22) so that there are
at least two "pages" per chunk.  In practice this isn't a particularly
compelling configuration because so much (unusable) virtual memory is
dedicated to chunk headers.

src/arena.c

@@ -2500,7 +2500,7 @@ arena_palloc_large(tsd_t *tsd, arena_t *arena, size_t usize, size_t alignment,
 		return (NULL);
 
 	alignment = PAGE_CEILING(alignment);
-	alloc_size = usize + large_pad + alignment - PAGE;
+	alloc_size = usize + large_pad + alignment;
 
 	malloc_mutex_lock(&arena->lock);
 	run = arena_run_alloc_large(arena, alloc_size, false);

src/chunk_mmap.c

@@ -9,7 +9,7 @@ chunk_alloc_mmap_slow(size_t size, size_t alignment, bool *zero, bool *commit)
 	void *ret;
 	size_t alloc_size;
 
-	alloc_size = size + alignment - PAGE;
+	alloc_size = size + alignment;
 	/* Beware size_t wrap-around. */
 	if (alloc_size < size)
 		return (NULL);