Remove PAGE_SIZE call sites.

To enable experiments with non-4KiB page sizes, introduce
an inline page_size() function that will either return the runtime
page size (if PAGE_SIZE is not 4096) or a constant 4096 (elsewhere).
This should ensure that there are no changes to the generated code on
unaffected platforms.

Test: source build/envsetup.sh
      lunch aosp_cf_arm64_16k_phone-userdebug
      m -j32 installclean
      m -j32
Test: launch_cvd \
  -kernel_path /path/to/out/android14-5.15/dist/Image \
  -initramfs_path /path/to/out/android14-5.15/dist/initramfs.img \
  -userdata_format=ext4
Bug: 277272383
Bug: 230790254
Change-Id: Ic0ed98b67f7c6b845804b90a4e16649f2fc94028
diff --git a/libc/bionic/mremap.cpp b/libc/bionic/mremap.cpp
index d7c9353..88ec829 100644
--- a/libc/bionic/mremap.cpp
+++ b/libc/bionic/mremap.cpp
@@ -33,12 +33,13 @@
 #include <unistd.h>
 
 #include "platform/bionic/macros.h"
+#include "platform/bionic/page.h"
 
 extern "C" void* __mremap(void*, size_t, size_t, int, void*);
 
 void* mremap(void* old_address, size_t old_size, size_t new_size, int flags, ...) {
   // prevent allocations large enough for `end - start` to overflow
-  size_t rounded = __BIONIC_ALIGN(new_size, PAGE_SIZE);
+  size_t rounded = __BIONIC_ALIGN(new_size, page_size());
   if (rounded < new_size || rounded > PTRDIFF_MAX) {
     errno = ENOMEM;
     return MAP_FAILED;