Replace usage of PAGE_SIZE

Per RFC 0016[1], page size in userspace should be a runtime
determination and this constant should go away.

[1] https://fuchsia.dev/fuchsia-src/contribute/governance/rfcs/0016_boot_time_page_sizes

Bug: 42146863
Change-Id: I09aabf29a24d3124b3d914fa86242d3c941297d6
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/android.googlesource.com/platform/system/libfmq/+/1421424
Reviewed-by: James Robinson <jamesr@google.com>

tests/fmq_unit_tests.cpp

diff --git a/tests/fmq_unit_tests.cpp b/tests/fmq_unit_tests.cpp
index c6b72ce..7b1ae5a 100644
--- a/tests/fmq_unit_tests.cpp
+++ b/tests/fmq_unit_tests.cpp
@@ -29,6 +29,8 @@
 #include <thread>
 
 #if defined(__Fuchsia__)
+#include <zircon/syscalls.h>
+
 #include "vendor/google/starnix/android/remote_binder/lutex.h"
 #endif
 
@@ -164,9 +166,10 @@
   public:
 #if defined(__Fuchsia__)
     RegisteredFutex() {
+        const size_t page_size = zx_system_get_page_size();
         android::base::unique_fd fd(::ashmem_create_region(
-            "RegisteredFutex", PAGE_SIZE));
-        mapping = mmap(0, PAGE_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd.get(), 0);
+            "RegisteredFutex", page_size));
+        mapping = mmap(0, page_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd.get(), 0);
         if (mapping != MAP_FAILED) {
             starnix::register_lutex(fd.get(), 0, mapping);
         }
@@ -174,7 +177,7 @@
 
     ~RegisteredFutex() {
       starnix::unregister_lutex(mapping);
-      munmap(mapping, PAGE_SIZE);
+      munmap(mapping, zx_system_get_page_size());
     }
 #else
     RegisteredFutex() {