Fix a crash of racket on OpenBSD systems, when pixman is compiled with SSE support and racket tries to load cairo using libffi.

Bug found initially by Juan Francisco Cantero Hurtado. Reported by many.

Mark Kettenis (from the OpenBSD Project) found the real reason of the
crash and created this patch for libffi. Patch taken from OpenBSD Ports.

racket/src/foreign/README

@@ -23,6 +23,8 @@ Local changes:
  * In "x86/ffi.c" for Win32, don't try to align stack for
    stdcall, fastcall, or thiscall.
 
+ * In "x86/freebsd.S", align the stack pointer to 16 bytes.
+
 ========== libffi_msvc directory =====================================
 
 This directory is synced from the CTypes repository

racket/src/foreign/libffi/src/x86/freebsd.S

@@ -49,6 +49,9 @@ ffi_call_SYSV:
 	movl  16(%ebp),%ecx
 	subl  %ecx,%esp
 
+	/* Align the stack pointer to 16-bytes */
+	andl  $0xfffffff0, %esp
+
 	movl  %esp,%eax
 
 	/* Place all of the ffi_prep_args in position  */