Add auxv entry for usrstack on FreeBSD 14, part 1

Previously the user stack was obtained using the kern.usrstack
sysctl. This has been moved to auxv in FreeBSD 14. Without
this change all programs linked with libthr fail with a panic
when they fail to get a valid user stack address.

Note also in FreeBSD 14 ASLR has been enabled. This means that
there is now some extra difference between the address layout of
a standalone executable and the same executable under valgrind.

Pre-FreeBSD 14 and under valgrind:
lib rtld is loaded after the executable (though a much smaller gap
inder valgrind)
user stack starts at 0x7ffffffff000

FreeBSD 14
lib rtld is loaded at a much higher address, around 0xeeeecc15000
user stack is at a much lower address, around 0x82073d000

This means that valgrind behaves somewhat as thogh the
kern.elf(64|32).aslr.stack sysctl were set to 0.

Some more work will be needed for the stack size.
There are no plans at the moment to match the FreeBSD 14 memory
layout.

coregrind/m_initimg/initimg-freebsd.c

@@ -713,6 +713,15 @@ Addr setup_client_stack( void*  init_sp,
       // case AT_KPRELOAD:
 #endif
 
+#if (FREEBSD_VERS >= FREEBSD_14)
+      case AT_USRSTACKBASE:
+         auxv->u.a_val = VG_(get_usrstack)();
+         break;
+      case AT_USRSTACKLIM:
+         auxv->a_type = AT_IGNORE;
+         break;
+#endif
+
       case AT_PHDR:
          if (info->phdr == 0)
             auxv->a_type = AT_IGNORE;