bionic: Adapt segment gap test for various page sizes

This makes the segment gap test compatible with systems having different
page sizes.

- The linker script now uses `ALIGN(CONSTANT (MAXPAGESIZE))`.
- The C++ code now uses `sysconf(_SC_PAGESIZE)` to get the actual page
  size ensuring correct memory alignment of 'start_of_gap' at runtime.

Bug: 315509500
Test: (16k testing)
      lunch aosp_cf_x86_64_phone_pgagnostic-trunk_staging-userdebug
      m && launch_cvd
      atest dlfcn#segment_gap

      (4k testing)
      lunch aosp_cf_x86_64_phone-trunk_staging-userdebug
      m && launch_cvd
      atest dlfcn#segment_gap

Change-Id: I31d45d1d22a237ef7d5ea487a64ef20e5e984ec0

Author: Vilas Bhat

tests/libs/segment_gap_outer.cpp

@@ -1,6 +1,7 @@
 #include <android/dlext.h>
 #include <dlfcn.h>
 #include <stdlib.h>
+#include <unistd.h>
 
 extern "C" void __attribute__((section(".custom_text"))) text_before_start_of_gap() {}
 char __attribute__((section(".custom_bss"))) end_of_gap[0x1000];
@@ -10,8 +11,9 @@ extern "C" void* get_inner() {
   info.flags = ANDROID_DLEXT_RESERVED_ADDRESS;
 
   char* start_of_gap =
-      reinterpret_cast<char*>(reinterpret_cast<uintptr_t>(text_before_start_of_gap) & ~0xfffull) +
-      0x1000;
+      reinterpret_cast<char*>(
+          (reinterpret_cast<uintptr_t>(text_before_start_of_gap) &
+           ~(sysconf(_SC_PAGESIZE) - 1)) + sysconf(_SC_PAGESIZE));
   info.reserved_addr = start_of_gap;
   info.reserved_size = end_of_gap - start_of_gap;
 

tests/libs/segment_gap_outer.lds

@@ -3,17 +3,17 @@ SECTIONS {
   # appropriate alignment between them.
   . = SIZEOF_HEADERS;
   .rodata : {*(.rodata .rodata.*)}
-  . = ALIGN(0x1000);
+  . = ALIGN(CONSTANT (MAXPAGESIZE));
   .text : {*(.text .text.*)}
-  . = ALIGN(0x1000);
+  . = ALIGN(CONSTANT (MAXPAGESIZE));
   .dynamic : {*(.dynamic)}
-  . = ALIGN(0x1000);
+  . = ALIGN(CONSTANT (MAXPAGESIZE));
   .data : {*(.data .data.*)}
   .bss : {*(.bss .bss.*)}
 
   # Now create the gap. We need a text segment first to prevent the linker from
   # merging .bss with .custom_bss.
-  . = ALIGN(0x1000);
+  . = ALIGN(CONSTANT (MAXPAGESIZE));
   .custom_text : {
     *(.custom_text);
   }