Workaround 'unexpected bus error at 0' abort on Serenity OS

(fix of commit b48ee2b)

PR #689 (bdwgc).

For an unknown reason, enabling of incremental mode based on `mprotect`
causes a null pointer dereference in `GC_finish_collection()`.

This patch removes `define MPROTECT_VDB` for Serenity OS.

* include/private/gcconfig.h [SERENITY] (MPROTECT_VDB): Do not define.
* os_dep.c [MPROTECT_VDB && !DARWIN && !MSWIN32 && !MSWINCE
&& SERENITY] (CODE_OK): Likewise.
* os_dep.c [MPROTECT_VDB && !DARWIN && !USE_WINALLOC && !AIX
&& !CYGWIN32 && !HAIKU]: Include `sys/syscall.h` regardless of
`SERENITY`.

include/private/gcconfig.h

@@ -1083,7 +1083,7 @@ extern int etext[], _end[];
 #  define DATASTART PTR_ALIGN_UP((ptr_t)etext, 0x1000)
 #  define DATAEND ((ptr_t)_end)
 #  define DYNAMIC_LOADING
-#  define MPROTECT_VDB
+/* TODO: enable mprotect-based VDB */
 #  define USE_MMAP_ANON
 #endif /* SERENITY */
 

os_dep.c

@@ -3259,8 +3259,7 @@ STATIC mach_port_t GC_task_self = 0;
 
 #  elif !defined(USE_WINALLOC)
 #    include <sys/mman.h>
-#    if !defined(AIX) && !defined(CYGWIN32) && !defined(HAIKU) \
-        && !defined(SERENITY)
+#    if !defined(AIX) && !defined(CYGWIN32) && !defined(HAIKU)
 #      include <sys/syscall.h>
 #    endif
 
@@ -3377,8 +3376,7 @@ is_header_found_async(const void *p)
 #      elif defined(IRIX5)
 #        define CODE_OK (si->si_code == EACCES)
 #      elif defined(AIX) || defined(COSMO) || defined(CYGWIN32) \
-          || defined(HAIKU) || defined(HURD) || defined(LINUX)  \
-          || defined(SERENITY)
+          || defined(HAIKU) || defined(HURD) || defined(LINUX)
 /* Linux: Empirically c.trapno == 14, on IA32, but is that useful?      */
 /* Should probably consider alignment issues on other architectures.    */
 #        define CODE_OK TRUE