Use madvise() to return memory to OS on Cosmo

Issue #666 (bdwgc).

Optionally, if -D FORCE_MPROTECT_BEFORE_MADVISE is passed to
CFLAGS_EXTRA, mprotect(PROT_NONE) is called before
madvise(MADV_DONTNEED).

* os_dep.c [USE_MUNMAP && !USE_WINALLOC && !SN_TARGET_PS3 && COSMO]
(block_unmap_inner): Call madvise(MADV_DONTNEED) instead of
mmap(PROT_NONE).
* os_dep.c [USE_MUNMAP && !USE_WINALLOC && !SN_TARGET_PS3 && COSMO
&& FORCE_MPROTECT_BEFORE_MADVISE] (block_unmap_inner): Call
mprotect(PROT_NONE).
* os_dep.c [USE_MUNMAP && !USE_WINALLOC && !SN_TARGET_PS3
&& !FORCE_MPROTECT_BEFORE_MADVISE&& COSMO] (GC_remap): Do nothing
instead of mprotect(PROT_READ|PROT_WRITE).
* os_dep.c [USE_MUNMAP && !USE_WINALLOC && COSMO
&& FORCE_MPROTECT_BEFORE_MADVISE] (GC_remap): Call
mmap(PROT_READ|PROT_WRITE).

os_dep.c

@@ -2703,17 +2703,18 @@ block_unmap_inner(ptr_t start_addr, size_t len)
   if (len != 0) {
 #    ifdef SN_TARGET_PS3
     ps3_free_mem(start_addr, len);
-#    elif defined(AIX) || defined(CYGWIN32) || defined(HAIKU)  \
-        || (defined(LINUX) && !defined(PREFER_MMAP_PROT_NONE)) \
-        || defined(HPUX)
+#    elif defined(AIX) || defined(COSMO) || defined(CYGWIN32) \
+        || defined(HAIKU) || 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.             */
     /* On Cygwin, calling mmap() with the new protection flags on */
     /* an existing memory map with MAP_FIXED is broken.           */
     /* However, calling mprotect() on the given address range     */
     /* with PROT_NONE seems to work fine.                         */
     /* On Linux, low RLIMIT_AS value may lead to mmap failure.    */
-#      if defined(LINUX) && !defined(FORCE_MPROTECT_BEFORE_MADVISE)
+#      if (defined(COSMO) || defined(LINUX)) \
+          && !defined(FORCE_MPROTECT_BEFORE_MADVISE)
     /* On Linux, at least, madvise() should be sufficient.      */
 #      else
     if (mprotect(start_addr, len, PROT_NONE))
@@ -2807,9 +2808,10 @@ GC_remap(ptr_t start, size_t bytes)
   /* It was already remapped with PROT_NONE. */
   {
 #    if !defined(SN_TARGET_PS3) && !defined(FORCE_MPROTECT_BEFORE_MADVISE) \
-        && defined(LINUX) && !defined(PREFER_MMAP_PROT_NONE)
+        && (defined(LINUX) && !defined(PREFER_MMAP_PROT_NONE)              \
+            || defined(COSMO))
     /* Nothing to unprotect as madvise() is just a hint.  */
-#    elif defined(NACL) || defined(NETBSD)
+#    elif defined(COSMO) || defined(NACL) || defined(NETBSD)
     /* NaCl does not expose mprotect, but mmap should work fine.  */
     /* In case of NetBSD, mprotect fails (unlike mmap) even       */
     /* without PROT_EXEC if PaX MPROTECT feature is enabled.      */