Don't use madvise() on SerenityOS

SerenityOS only supports MADV_SET_VOLATILE/MADV_SET_NONVOLATILE and
returns EINVAL for MADV_DONTNEED:
https://github.com/SerenityOS/serenity/blob/b88cd185a0ec40fc10405b555507aa6f0aab8222/Kernel/Syscalls/mmap.cpp#L355-L388

See: #688

os_dep.c

@@ -2770,7 +2770,7 @@ block_unmap_inner(ptr_t start_addr, size_t len)
 #    ifdef SN_TARGET_PS3
     ps3_free_mem(start_addr, len);
 #    elif defined(AIX) || defined(COSMO) || defined(CYGWIN32) \
-        || defined(HPUX) || defined(SERENITY)                 \
+        || defined(HPUX)                                      \
         || (defined(LINUX) && !defined(PREFER_MMAP_PROT_NONE))
     /* On AIX, mmap(PROT_NONE) fails with ENOMEM unless the       */
     /* environment variable XPG_SUS_ENV is set to ON.             */