Bug 648735 - Fix build failure on OpenBSD/i386 & macppc; r=cjones

../../ipc/chromium/src/base/singleton.h:171: error: invalid conversion
from 'base::subtle::AtomicWord*' to 'volatile base::subtle::Atomic32*'

Lifted from www/chromium/patches/patch-base_atomicops_h

ipc/chromium/src/base/atomicops.h

@@ -45,7 +45,11 @@ typedef int64 Atomic64;
 
 // Use AtomicWord for a machine-sized pointer.  It will use the Atomic32 or
 // Atomic64 routines below, depending on your architecture.
+#if defined(OS_OPENBSD) && !defined(ARCH_CPU_64_BITS)
+typedef Atomic32 AtomicWord;
+#else
 typedef intptr_t AtomicWord;
+#endif
 
 // Atomically execute:
 //      result = *ptr;